The development of some Silurian Brachiopoda : with eight plates

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University of the State of New York

MEMOLIRS

OF THE

NEW YORK STATE MUSEUM

VOL. I. NO. 1. OCT., 1889

THE DEVELOPMENT OF SOME SILURIAN BRACHIOPODA

WITH EIGHT PLATES

CHARLES E, BEECHER JOHN M. CLARKE
A

CONSULTING PALBONTOLOGIST OF THE STATE MUSEUM ASSISTANT STATE PALE@GNTOLOGIST

ALBANY
UNIVERSITY OF THE STATE OF NEW YORK
1889

TABLE OF CONTENTS

TITLE PAGE, a PE Oe eae ST. A
TABLE OF CONTENTS, we \ Fata 2
INTRODUCTION, .. eee Ae.
DISCUSSIONS OF THE SPECIES,

CRANIA SILURIANA, Hall, 2 5 E :
ORTHIS ELEGANTULA, Dalman,

ORTHIS HYBRIDA, Sowerby, -
STROPHOMENA RHOMBOIDALIS, Wilckens, P 3

STREPTORHYNCHUS SUBPLANUM, Conrad,
STROPHONELLA STRIATA, Hall, : ;

STREPTIS WALDRONENSIS, Miller and Dyer,

EICHWALDIA RETICULATA, Hall,

ANASTROPHIA INTERNASCENS, Hall,

RHYNCHONELLA AcINus, Hall,

RHYNCHONELLA NeGLECTA, Hall, . . .

RHYNCHONELLA wuHiTII, Hall,

RHYNCHONELLA INDIANENSIS, Hall, : .

RHYNCHOTRETA CUNEATA, (Dalman) var. AMERICANA, Hall,
' ATRYPA RETICULARIS, Linneus, 2

Revtzia EVAX, Hall, . ; - + Mee

RETZIA SOBRINA, Sp. 0.,

CLOsPIRA DISPARILIS, Hall, é ; 5

MERISTELLA RECTIROSTRA, Hall, : > A 7

MERISTINA NiTIDA, Hall,

WHITFIELDIA MARIA, Hall,

SprriFER Crispus, Hisinger, Saaee (AC SN

SPIRIFER CRISPUS, var. SIMPLEX, Hall,

SprrireR BICosTaTUs, (Vanuxem) var. PETILUs, Hall,

SPIRIFER RADIATUS, Sowerby, <
\RY OF DEVELOPMENTAL CHANGES, a ; 2
MANINEXPLANATIONS «2 2. «© . «. «©. I-VIII

THE DEVELOPMENT OF SOME SILURIAN BRACHIOPODA

By Joun M. Crarke and Cuaries E, BEECHER*

INTRODUCTION

The fossil faunas of our rock systems rarely furnish material for trac-
ing the individual development of any of the contained species. Much
will doubtless be done toward ascertaining such development when large
collections from suitable localities have been studied with this object in
view, and when the number of new species discovered and described each
year approaches a minimum. We have a comparatively full and satisfac-
tory account of the development of the individual organism in several
species of trilobites, as given in the works of BarranpE, WaLcort, Forp
and Matruew; Hyatt, Branco, Moyjsisovics and others have demon-
strated the developmental characters of many of the fossil cephalopods,
and Verworn has elicited similar facts from certain extinct species of
ostracoda. ‘Farther than this but little has been attempted, although the
field is a most extensive, important, and inviting one.

As a general rule, the treatment of fossil organisms has rested mainly

*The order of the names of the authors of this paper is without significance. The work
was equally divided and jointly reviewed.

6 INTRODUCTION

with geologists having more or less of a zoélogical training, and the prin-
cipal aim has been to present the faunal aspects of each horizon for the
purpose of chronological identification. This process has frequently
become so involved with the imperfect description of species, that the
systematic zoélogist or paleontologist is unable to make any use of a large
proportion of the species as a means of studying their taxonomic relations
or their structural affinities with each other and with recent forms.

Each revision of a group of fossil animals has resulted in the establish-
ment of numerous specific and generic synonyms. Many of these are
owing, of necessity, to the imperfection of the material, and many names
which are finally relegated as synonyms, have been created under a mis-
conception of the full significance of age, sex, habitat and condition of
preservation. Additional confusion often results from the inclusion, in a
generic or specific description, of characters which pertain not alone to a
normal individual, but interspersed with certain normal adult features are
those belonging to various stages of morphological development and pe-
culiarities arising from accident, disease and impoverished conditions.

In the case of rare species, or of meager material belonging to common
forms, it is to be noticed that assertions regarding specific and generic
characters are usually very positive; while, with an abundance of speci-
mens representing many stages of growth and the extremes of individual
variation, the descriptions are qualified, the latitude of genera and species
is extended, and the points of relationship with allied forms are multiplied,
thus binding a group of organisms into comparative uniformity, without
anomalous differences such as often occur where the dividing lines are
rigidly drawn.

During the years 1878-79, the collection of fossils made from the
Niagara group at Waldron, Indiana, for the New York State Museum,
was studied and arranged by one’ of the writers. This is probably the

largest collection yet brought together from that celebrated locality, and
- . .

a " oat al ..

INTRODUCTION 7

some conception of its size may be obtained from the fact that, when re-
ceived, it weighed about seven tons. At the time mentioned, all the
mature specimens were selected and specifically separated. Many imma-
ture forms were also reserved and used in arranging the series pre-
pared for exhibition in the State Museum. It was designed to represent
in the arrangement, each species by a series of specimens showing the
gradations of size and form from mature individuals down to as young and
small specimens as could be found. Abnormal examples, also, were
reserved and grouped with them. It was the intention of the writers to
accompany this memoir with photographic illustrations of these series,
representing each species here discussed, but it has not been found wholly
feasible, and the illustrations are largely restricted to the presentation
of the immature and adult conditions of growth, with the exception of the
several series which are given on plate 8.

The product obtained from washing the slabs, was preserved and
passed through sieves to assort the material into different grades of fine-
ness. It was found that these washings contained a great number of
partially developed shells, and it is from them that the extremely young
brachiopods, treated of in the present paper, have been derived. The
writers have carefully examined all the residue of these washings and have
picked out about fifty thousand specimens, most of which are less than five
millimeters in length, and many have a length of not more than one milli-
meter. .After all the imperfect and badly preserved individuals were
rejected, there still remained more than fifteen thousand inchoate indi-
viduals.

The sediments at Waldron consist of fine calcareous shales, weathering
into clays. A stratum of Niagara limestone overlies the shales at this
locality, but none of the fossils derived from this limestone have been used
in the preparation of the present paper, and so far as known, it has a com-
paratively different fauna and does not furnish such material as is here

8 INTRODUCTION

described. The calcareous matter in the shales consists almost entirely
of fossils and fragments of fossils, principally branches of corals and
bryozoa, segments of crinoid columns, and broken crinoid plates. The
brachiopoda are all calcareous and the original shell structure is more or
less preserved, depending upon the absence or presence of pyrite.

The occurrence in such great numbers of immature shells in these
deposits may be explained by the luxuriant fauna which flourished in this
Niagara basin, by the quiet seas of this region and by the rapid sedimenta-
tion of the shales. The richness of the material is shown by the great
profusion of specimens representing the sponges, corals, crinoids, bryozoa,
brachiopods, gastropods, annelids and crustaceans, comprising altogether
about one hundred and fifty species. The lamellibranchs and cephalopods
were also doubtless abundant, but the conditions existing for the preserva-
tion of their remains were not favorable, probably on account of the com-
position of their shells, and but sixteen species have been noted. That the
fauna was protected from excessive storms and the’ action of sea currents, is
evinced by the usual perfection of the fossils. Some of the crinoids are
unbroken and remain attached by their roots, retaining their arms in place ;
also, large colonies of delicate branching corals and bryozoa still preserve
their unity. The specimens.were rapidly buried in the soft calcareous mud
and show none of the eroding or disintegrating action of the water, such as
would have been produced had they lain for any considerable period
unprotected on the sea bottom. It is true that many specimens are
incrusted with bryozoa, annelids, cranias and other fixed and incrusting
forms, but the majority of these seem to have flourished during the life
of their hosts. -

Besides the embryonic brachiopoda occurring in these shales, there are ~
other classes represented by immature forms, notably the gastropoda and
crinoidea. These, with the brachiopoda, embrace almost all the young
forms found, The small gastropods are of little interest, on account of the

INTRODUCTION 9

limited number of species, and because they undergo no important modi-
fication in their subsequent growth, and merely represent the apical portion
of mature individuals. Among the crinoids, the modifications of form and
structure from the embryo state to maturity are more profound and essen-
tial, although the material is not sufficiently complete to furnish any very
important results.

It is necessary to state that nearly all our observations on the develop-
ment of the brachiopoda are based upon the study of the material derived
from a single locality, and some of the minor deductions may not apply, in
every case, to the individuals of the same species found in other regions.
The writers have also refrained, except when essential to the proper expo-
sition of a species, from entering into details of synonymy or generic contro-
versy as to the correct reference of the species. This course is considered
advisable, from a desire not to introduce any discussions alien to the
descriptions of the developmental changes in these organisms. Aside from
this, it is believed that a number of important facts are here added to the
knowledge of the brachiopoda, and that many of them will be found to be
of general application. The investigation has also resulted in elucidating
several obscure and anomalous features of the shell and of the cardinal
area, which appear in their proper place in the description of the spe-
cies and in the general summary.

The following list includes all the species of brachiopoda which, up to
this time, have been described from the shales at Waldron, Indiana, and
comprises forty-two species and varieties, ascribed to twenty-four genera.
It also shows whether material has been obtained which furnishes data
for tracing the developmental changes.

The majority of the species which have afforded no young specimens
are rare forms even in their adult state. Among the actually abundant
species of which there are no means accessible of tracing the life-history,

Riynchonella stricklandi is a noticeable example, and it is- really the only
2

10 INTRODUCTION

common species which has afforded no young shells. Waztfeldia maria,
another abundant form, furnishes a series which is notably incomplete, as
the youngest individual observed, which can with certainty be referred to it,
has a length of 6 mm. Likewise, the inarticulate species have yielded
almost no immature specimens.

List OF THE BRACHIOPODA OCCURRING IN THE NIAGARA SHALES AT
Watpron,, INDIANA

Crantia siluriana, Hall,

Crania setifera, Hall,

Crania spinigera, Hall,

Lingula gibbosa, Hall,

Pholidops ovals, Hall,

Orthis hybrida, Sowerby,

Orthis elegantula, Dalman,

Orthis subnodosa, Hall, .

Orthis biloba, Linnzus,
Streptorhynchus tenue, Hall,
Streptorhynchus subplanum, Conrad,
Strophomena rhombotdalis, Wilckens,
Strophodonta profunda, Hall,
Strophonella striata, Hall,
Strophonella semifasciata, Hall, .
Leptena transversalis, Dalman,
Streptis waldronensis, Miller and Dyer,
Chonetes nova-scotica, Hall,

Chonetes undulata, Hall,
Eichwaldia reticulata, Hall, .
Pentamerus fornicatus, var., Hall,
Anastrophia internascens, Hall,
Rhynchonella neglecta, Hall,
Rhynchonella acinus, Hall,
Rhynchonella indianensis, Hall,
Rhynchonella whiti, Hali,
Rhynchonella stricklandi, Sowerby,
Rhynchotreta cuneata, Dalman,

One embryo.
No young shells obtained.

“ “cc

Numerous inchoate specimens.
“ce “ce

No young shells obtained.
ee ai
“c “
Full series, showing development.
“ee ae
No young shells obtained.
Full series, showing development.
No young shells obtained.
“ee ae
One embryo,
No young shells obtained.
“ ia
Young shells not rare.
No young shells obtained.
Young shells not rare.
Young shells very abundant.

Mature form abundant; no young shells obtained.
Young shells common. >

INTRODUCTION 11
Atrypa reticularzs, Linneus, : . Young shells abundant.
Zygospira minima, Hall, : - : No young shells obtained.
Celospira disparilis, Hall, . é : . Young shells common.
Retzia evax, Hall, : i : : Young shells very abundant.
Retzia sobrina, sp. n. - é . Young shells not rare.
*Nucleospira pistformzs, Hall, A : Ss $f
Meristella rectirostra, Hall, : ; . Full series, showing development.
Meristina nitida, Hall, . F : - Numerous inchoate specimens.
Whitfieldia marta, Hall, . ; ‘ . Incomplete series showing development.
Spirifer eudora, Hall, . : f é No young shells obtained.
Spirifer crispus, Hisinger, ‘ . Full series, showing development.

Spirifer crispus, var. simplex, Hall, : a ee
Spirifer radiatus, Sowerby, ; P - ce ES
Spirifer bicostatus, var. petilus, Hall, 2 as ie

The method of illustration which has been adopted is one which
seems most readily to furnish a means for comparison of characters. The
embryonic shells are represented as enlarged, usually to the Size of an adult,
and accompanying the enlargements are natural size representations of the
final result of normal growth. Where the mature forms have been too
minute to show satisfactorily the details of structure, both the develop-
mental stages and full grown shell have been enlarged to a convenient size.
Thus the incipient stages and mature specific form are presented together.
In the delineation of special features, such as the hinge, we have sometimes
enlarged the earlier phases to a size corresponding with the same structure
in the mature form, or have increased all on a uniform scale, so that both
the particular characters and their comparative size are presented.

The enlarged drawings have been made by the writers, principally
from the microscope; the camera lucida was employed to ensure accuracy
in outline. The illustrations of the mature specimens are largely taken
from the Pmenty- eighth Annual meron of oe New York State Museum

*The mature charactors of this species are Ausuitiod so véacly, that the youngest teriad ob-
served show no important differences from the adult. On this account, no discussion of its
characters is given in the ensuing pages.

12 INTRODUCTION

and from the Eleventh Report of the State Geologist of Indiana, which
may be consulted for a more ample representation of the adult characters
of the species occurring at Waldron.

The drawings on plates 1 to 7 have been reproduced on stone, in a
most satisfactory manner, by Mr. Puitie Ast, and we wish to express our
appreciation of the skill and labor he has bestowed upon the work. The
illustrations given on plate 8 were made from photographic reproductions
of the actual series of specimens, and, although not serviceable for purposes
of detailed study, show distinctly the nature of the material used and the
almost insensible gradations obtained, representing the life-history of these
species. The same completeness of material is furnished by the majority
of forms described in the following pages.

The arrangement of the subject-matter in the discussions of the species
may not seem to be in accordance with the usual method employed in trac-

ing the life-history of organisms. In this case we are dealing with fossil
organisms and, in order to insure accuracy of results, it is necessary to

begin with the known and established facts and gradually descend to
to minute and strange forms, thereby connecting the extremes of growth.
Under the caption, ‘“ Developmental Changes,” we have, however, endeav-
ored to trace the history of each feature of the shell, from its inception to

maturity.

ml ar

DISCUSSIONS OF THE SPECIES

CRANIA SILURIANA, Hall, 1863
PiatTeE I, Fics. 1, 2

— ——, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 148, pl. 21, figs. 3-7.

1879.
—— -—, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 282, pl. 21, figs. 3-7. 1882.

But a single embryo of this species has been found in all the material
examined. It is of an incipient stage of growth, measuring but 1 mm.
in height, and 1.5 mm. across the aperture. Compared with the mature
form, the average size of which is about 9x20 mm., it shows a relatively
greater elevation and a more regularly conical form. Otherwise, all the
few essential characters of the adult shell are present at this early age.

°

OrTHIS ELEGANTULA, Dalman, 1827
Piate I, Fics, 3-11

—— —, Hall. Twenty-eighth Rept. N. Y. State Mus. Nat. Hist., p. 150, pl. 21, figs. 11-17. 1879.
—— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 285, pl. 21, figs. 11-17. 1882.

Both the species of Orthzs occurring at Waldron (O. elegantula and
O. hybrida) are very abundant. Inthe later stages of growth, the former
species is readily distinguished from the latter by its flatter and shallower
dorsal valve and deeper ventral valve, features which usually hold good for
purposes of discrimination ; but in extremely early stages of growth, the
nearly equivalve form of the shell makes the separation of the species very

14 MEMOIRS OF THE STATE MUSEUM

difficult, perhaps even impossible. Between the dimensions of .5x.75 mm,
and 18.5x18 mm. (which is a little in excesseof average mature size), we
have found every gradation in size and development. The minute shell
which serves as a starting point for the series may quite as well be taken
as the incipient shell of Orthzs hydrzda, as both its valves have the same
depth, while the cardinal areas and beaks show the same character of
development. As there can be no doubt of this fact, it becomes impossible
to determine whether a given embryo, could it have grown to maturity,
would have developed into O. hyébrzda or O. elegantula. Until the embryos
reach a size of 2 or 2.5 mm. in length, their specific value is undeterminable,
and the specific individuality of O. edegantula can be established only with
the increasing depth of the ventral valve from this point upward toward
adolescence. :

Unless our observations are at fault, and they have been made with *
great care, evidence here is very positive, that the diagnostic characters of
species of this genus may not be assumed until the earlier stages of the
existence of the shell have passed.. Indications of similar character are
found among the species of Rhynchonella and Sperzfer. The importance
of the fact is apparent and its significance will be appreciated.

SPECIFIC CHARACTERS

Mature Form (plate 1, figs. 10-12@). | Outline subcircular ; hinge
line short, about one-half the width of the shell, straight.

Ventral valve elevated along the dorsum, which is arched and slopes
more rapidly toward the lateral than toward the anterior margin ; greatest
width below the hinge-line, about half-way down the valve. Beak full,
arched, incurved, and projecting over the cardinal area sufficiently to conceal
the foramen. Cardinal area broadly triangular, low, incurved ; foramen
triangular ; deltidial plates absent.

Dorsal valve shallow, nearly flat, slightly rounded over the umbo, but
depressed toward the margins. A sharply defined sinus starts near the

J

»
:
‘
-

SILURIAN BRACHIOPODA 15

apex, but by widening with the growth of the shell, it becomes nearly obso-
lete before reaching the margins. Cardinal line straight ; cardinal area narrow,
elongate triangular ; beak inconspicuous. Foramen triangular and filled by
a tripartite cardinal process which passes into, without filling the foramen
of the opposite valve.

Surface of the shell closely covered by fine thread-like strize which
increase by intercalation; concentric growth-lines rare, except near the
margin where they appear as wrinkles.

Incipient Form (plate 1, figs. 3, 3a). The initial shell of our series,
measuring .5 mm. in length and .75 mm. in width, has valves of equal
depth and convexity. The length of the hinge-line nearly equals the great-
est width of the shell. The cardinal area is high, and equally elevated on
each valve. Beaks erect; foramina large, triangular, open and marginate.
On the ventra/ valve is a single median stria, representing the dorsum of
the mature shell, accompanied by one and indications of a second on each
of the lateral areas, making three (i. e., five) striae on the valve. On the
dorsal valve a low and wide median depression is apparent, bounded by two
central striz, these being accompanied by two accessory pairs upon the
latera, making six striz in all. It is very probable that this form represents
the actual initial stage in the development of the shell, and if this is the
case, the inception of the plications on the surface, which become so numer-

.ous at maturity from (from one hundred to one hundred and thirty on each

valve), is synchronous with the formation of the rudimentary shell, while in
the pauciplicate species here discussed they appear to be of secondary

growth.
DEVELOPMENTAL CHANGES

General Form and Outline. In the growth of the shell a change be-
comes manifest in its outline and relative proportions. The young stages
have the width greater than the length, but the more rapid axial growth of
the shell reverses these proportions in maturity, Moreover in the incipient
stages, the valves, as already noticed, are of nearly equal depth and con-

16 MEMOIRS OF THE STATE MUSEUM

vexity. In the next stage, the depth of the ventral valve has noticeably
increased over that of the dorsal, and, as in the latter valve the median sinus
has become distinctly developed, the difference in this respect becomes
emphasized. The divergence of the valves in convexity becomes increased
until maturity, and this growth is accompanied in the ventral valve by a
correspondingly increasing incurvature of the beak.

Beaks. In the incipient shell, the beaks are erect and distant, but not
prominent. By the development of the broad sinus on the dorsal valve, the
beak of this valve becomes relatively less prominent and apparently more
closely appressed to the cardinal line. On the opposite valve, every increase
in convexity is accompanied by a corresponding increase in the incurvature
of the beak, and as the shell approaches maturity, the incurvature becomes
so great, that it has been necessary, in the drawings which are here given
showing the features of the cardinal area, to represent the beak as broken
away.

Foramen. The earliest stages of growth show a remarkable feature in
the triangular, marginate, subequal fissures on the valves. This character
may prove of a high taxonomic value, as it clearly indicates the relation-
ship, in this respect, of Orthzs to the Linguloids, and places the genus near
the inarticulata. The foramen upon the ventral valve is, in every stage
of development, open and free for the protrusion of the pedicle. Del-
tidial plates are absent in every stage of growth. Ina secondary stage, a
cardinal process begins to form in the apex of the dorsal foramen, soon
widening and becoming tripartite. As age increases, this process is projected
into the ventral foramen, never quite filling it, always leaving room for the
protrusion of the pedicle. In immature conditions the cardinal process
is attached to the shell only at the apex of the foramen, but with ma-
turity it comes in contact with the sides of the foramen, and at this
stage entirely fills the dorsal aperture. With the increasing incurvature
of the ventral beak and cardinal area, the apertures of the two valves

——

SILURIAN BRACHIOPODA 17

change their mutual angle, constantly lessening it as growth ad-
vances. ;

Plications. As noticed above, the earliest stages of growth observed
show the striz to be already developed on the shell, five on the ventral
and six on the dorsal valve. These plications are rapidly multiplied by
interstitial addition, and in maturity number from one hundred to one
hundred and thirty on each valve.

OrTHIS HyBRIDA, Sowerby, 1839
PiateE I, Fics. 13-18

—— — , Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 149, pl.. 21, figs.
18-25. 1879.
—— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 285, pl. 21, figs. 18-25. 1882.

Orthis hybrida passes through primary developmental stages which
are essentially identical with those already described for O. elegantula.
Sufficient has been said in that connection in regard to the similarity and
probable identity of the earlier embryonic stages of the shell of both spe-
cies, the origin of the entire specific difference which is so apparent in the
later and mature periods of development lying in the unequal growth of the
valves in convexity. This increase is relatively greater in the dorsal
valve of O. hyérida than in that of O. e/egantula, and less in the ventral
valve of the former than in that of the latter species. Thus O. hybrida
is a more discoid, lenticular shell, showing but slight evidence of a median
fold and sinus and carrying on its surface at maturity just about as many
plications or striz as its associate.

There is an obese variation from the normal form of O. hydrida,
which was noticed by Professor Haut (/oc. cz¢.), and this appears early in
the development of the species, with a size of 3.5 mm. in length and 4.5 mm.
in width, and reaches a maximum growth with dimensions of 14x13 mm.
This variation is due to internal thickening and increase in convexity,

and is accompanied by abundant concentric growth-lines which are as
3

18 MEMOIRS OF THE STATE MUSEUM

rare in the normal form as in O. elegantula. Our representative series
of this species affords variations between the following limits of size:
.5 mm. length x .75 mm. width (minimum), and 17 mm. length x 20 mm.
width (maximum).

STROPHOMENA RHOMBOIDALIS, Wilckens, 1769
Pate II, Fics. 1-13

—— —, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 151, pl. 22, figs.

4-10. 1879.
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 288, pl. 22, figs. 4-10. 1882.

This well known species, although extremely’abundant in the mature
state, is correspondingly rare in its undeveloped condition. The young
specimens which have been found are nearly all more or less broken, and
it is evident that while young, the shell was thin and delicate, consequently
few of their remains have been preserved. The series which has been
selected is, however, very complete in its representation of the distinct
phases of growth through which the individuals pass in their development —
from youth to maturity. The initial form, without radiating strie ; the
second phase, a shell radiatingly striate, without undulations; the third
state, striated and concentrically undulated, but without the angular
geniculation of the valves in front; and the last phase, with the full
form and characters of maturity, offer a series of changes, not often trace-
able in Silurian brachiopods.

The development of the characters of the hinge-area is also very sat-
isfactorily demonstrated and affords some interesting points of comparison
with certain forms of Streptorhynchus and Strophonella. These features
are noticed at the end of the description of the species, Strophonella
striata,

SPECIFIC CHARACTERS

Mature Form (plate 2, figs. 4, 4a, 10, 13). Shell semi-elliptical or

semicircular in outline.

SILURIAN BRACHIOPODA 19

Dorsal valve flat or slightly concave in the upper part, with the
marginal portions abruptly curved upwards in front; beak small, carrying
on its inner side, a large, prominent, triangular callosity, grooved along its
summit and nearly filling the area of the opposite valve.

Ventral valve usually convex in the upper part, becoming flat or
concave below, and with the marginal portion produced and abruptly bent
downwards, geniculating with the dorsal valve; beak small, usually per-
forated with a small circular foramen; hinge-line often 50 mm. in length,
equaling or greater than.the width of the shell below; cardinal extremities
twisted and often much extended; cardinal area narrow, edges parallel,
formed by both valves; deltidial area of the ventral valve, broadly triangu-
lar, occupied by the grooved callosity under the dorsal beak.

Surface marked by regular, rounded, radiating striae. From the beaks
to the curtain, or geniculated portion, the shell is ornamented with regular,
_ strong, concentric undulations or corrugations.

This species varies greatly in size and form, in the different horizons
and localities where it is found. In many places the mature shells are
about half the size of the specimens from Waldron.

Incipient Form (plate 2, figs. 1, 1@, 11). The smallest entire speci-
men yet detected has a length of 1.25 mm. The outline is semi-oval, with
the greatest width near the middle, and about one-fourth greater than the
length. Dorsal valve convex in the upper part, becoming concave toward
the front. The hinge-area of this valve is very narrow and linear, and
carries beneath the beak a small grooved callosity. Ventral valve convex,
sloping in all directions from near the foramen, around which the surface
is slightly depressed. There is also a depression extending along the middle
of the valve to the anterior margin. The place of the beak is occupied by
an exsert, conical pedicle-tube, which partly protrudes beyond the cardinal
margin of the valve and extends down to, and embraces the dorsal callosity.
Cardinal area of the ventral valve comparatively broad, narrowing rapidly

20 MEMOIRS OF THE STATE MUSEUM

from the pedicle-tube to the extremities. Surface smooth, except along
a narrow zone around the margin, which shows incipient radiating striz.

DEVELOPMENTAL CHANGES

The form of this species being somewhat complex, we may make a
convenient subdivision of the development of the shell into four stages,
which may be briefly characterized as follows:

rst Stage. Length of shells .4—-1 mm.; surface smooth.

2d Stage. Length 1-2 mm.; shell radiatingly striated, without undulations.

3@ Stage. Length 2-20 mm.; shell radiatingly striated, and concentrically
undulated. |

gth Stage. Length 20-30 mm., entire shell radiatingly striated, concen-
trically undulated in the upper part, abruptly produced
and geniculated in front.

The changes taking place in the form and character of the shell
from one stage to another can best be shown and used for comparison
in the following tabulation, where the conditions incident to each stage
of growth in the various parts of the shell are briefly described.

Development of Strophomena rhomboidatles

1st Stage, ad Stage. _ 3d Stage. 4th Stage.
| Initial. Infantile. Adolescent, Mature.
Size..............4 mm.—1 mm, in| 1 mm.—2 mm. in 2 mm.— 20 mm. in| 20mm.— 30 mm. in

length, .4 mm.—) length, 1.5mm.— length, 2.5 mm.— length, 40 mm.—
1.5 mm. in width. 2.5mm. in width. 40 mm, in width. 50 mm. in width.

Transversely semi-

MOUs ¥chet Transversely semi-

Longitudinally, Longitudinal]
oval; cardinalex- elliptical; cardi-- semi-elliptical; semi-elliptical;
tremitiesobtusely,| nal extremities) cardinal extremi-| cardinal extremi-

| angular. angular. ties angular, be- ties acutely angu-
coming produced, lar, extended and

not twisted. twisted,

ae

SILURIAN BRACHIOPODA

2I

Development of Strophomena rhomboidalis — Continued

Pedicle-tube....

and _ concentri-
cally undulate.

1st Stage. ad Stage. 3d Stage. 4th Stage.
Initial. Infantile. Adolescent. Mature.
Contour «».| Convex. Depressed convex.) Very slightly con-} Geniculate, making
vex. the shell high]
arched longitudi-
nally.

Dorsal valve....| Convex, concave) Concave,except on| Concave,except on| Flat or concave on
on the margin; the umbo. the umbo. the body of the
umbo prominent. shell; abruptly

produced and
curved upwards
around the mar-
gins.

Ventral valve...) Convex. Convex, semi-coni-| Convex in the up-| Convex in the up-
cal with the beak| per part, flat or| per part, flat or
at the apex. concave on the} concave in the

margin. middle, and ab-
ruptly bent down-
. wards below.
Surface.........| Smooth. Radiatingly striate. Radiatingly striate| Entire surface,

radiatingly stri-
ate, concentrical-
ly undulate in the
upper part only.

Ventral high; dor-
sal very slender.

Ventral high; dor-
sal very slender.

Ventral narrow;
dorsal narrow.

Both narrow, sub-
equal.

Exsert, full height

Not exsert, full

Nearly full height

Obsolescent or ob-

of the area. height of thearea.| of the area. solete.
-Foramen.... .. Present, circular,| Present, circular. | Present, circular. | Usually present.
elevated
Dorsal callosity.| Small, grooved. | Small, grooved. | Larger, grooved. | Very large and
deeply grooved.
s
Y 5

22 MEMOIRS OF THE STATE MUSEUM

Among the mature shells, the greatest variation is to be found in
the development of the anterior curtain, or geniculate and sloping
marginal area of the valves. In some specimens this is so excessively
déveloped, that the posterior, or concentrically undulated portion of the
ventral valve is at right angles to the plane of the margin. Also, in
many specimens the curtain is obscurely plicate, and the radiating striz
are often irregular and sometimes fasciculate, while on the upper part
of the valves these striz are very uniform in their arrangement. No speci-
mens have been noticed which are so strongly quadriplicate as those illus-
trated by Mr. Davinson, on plate 39 of the “ British Silurian Brachiopoda.”

Senile specimens usually have the valves very much thickened from
internal growth, and the margins show strong varices. It is noticeable
that nearly all the old shells are covered with a growth of cranias,
bryozoa, favosites, etc., and it is very difficult to free the shell from
this overgrowth. In consequence of this, many of the shells are scarcely
recognizable, and resemble agglomerations of bryozoa and corals, The
only other species of brachiopoda at this locality commonly thus over-
grown and involved, is Atrypa reticularis.

Strophomena rhomboidalis is cosmopolitan and has been discussed by
many authors who have shown its great variation and wide distribution.
So far as known, the youngest specimen heretofore figured is one repre-
sented by Mr. Davipson.* This is an individual belonging to the third
stage of development, having a length of nearly 6 mm. and a distinct
circular perforation of the beak.

* British Fossil Brachiopoda, Vol. III, Devonian and Silurian, pp. 283, 284, pl. 30, fig. 6.
The same. General Summary to the British Fossil Brachiopoda, p. 289.

—s

SILURIAN BRACHIOPODA 23

STREPTORHYNCHUS SUBPLANUM, Conrad, 1842
PLATE II, Fics. 14-20

—— —, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 151, pl. 21, figs. 26-33.
— is Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 288, pl. 21, figs. 26-33. 1882.

The series selected to represent the development of this species com-
prises fourteen specimens ranging from 1.5 mm. to 26.5 mm. in length. The
external features of form and surface ornaments are remarkably constant
from the young to the mature shells. There is, however, a slight progres-
sive modification in the relative convexity of the valves. The dorsal valve
of young and half-grown individuals is nearly flat, while the ventral is
moderately convex. In old specimens both valves are convex, with the
dorsal somewhat more so than the ventral. The most marked changes due
to advancing growth are those which take place in the hinge. Some men-
tion of these is made under the description of Strophonella striata, where it
is stated that the pedicle-tube retains its embryonic form and size nearly up
to maturity, after which it is obscured by the internal thickening of the
shell ; also, that the callosity under the beak of the dorsal valve uniformly
increases in size from the youngést forms to full-grown specimens.

SPECIFIC CHARACTERS

Mature Form (plate 2, figs. 15, 15@,17,20). | Shell semicircular or
semi-elliptical, depressed convex; hinge-line longer than the width of the
shell ; cardinal angles flat and extended.

Dorsal valve moderately and uniformly convex except at the cardinal
angles; umbo not defined ; beak small.

Ventral valve convex on the umbo, less convex below, and in many
specimens the marginal portion is flat or slightly concave ; beak small,
somewhat arched. Hinge-area nearly equal in both valves, usually appear-
ing as a deep angular groove along the cardinal margin. Under the beak
of the dorsal valve is a large triangular callosity, grooved on the inside,

24 MEMOIRS OF THE STATE MUSEUM

and nearly filling the fissure of the opposite valve. Deltidium of the ven-
tral valve broadly triangular, extending to just below the beak, and mar-
gined on each side by two narrow areas in the form of scalene triangles,
which may represent the deltidial plates of other genera. Beak imperforate.

Surface marked by from fifty to one hundred (according to the size of
the shell) regular, rounded striae, with equal interspaces, increasing in num-
ber by interstitial additions. The entire shell is also ornamented with
very fine, regular, sharp, concentric striz. A large specimen has a length
of 26 mm., and the width, measured along the hinge-line, is about 38 mm.

Incipient Form (plate 2, figs. 14, 14@). The smallest specimen
measures 1.5 mm. in length, by 2.3 mm. in width along the hinge-line. The
outline is semielliptical, with the cardinal angles slightly extended. Dorsal
valve concave in the upper part, and slightly convex below. Ventral valve
convex ; beak prominent, projecting beyond the hinge-line.

The hinge characters are not well preserved in this individual. The
first specimen in the ascending series which shows the hinge distinctly, has
a length of 2.25 mm., and will be described in the development of this part.

The surface of the incipient shell is marked by seventeen alternating,
narrow, elevated radiating lines, with wider interspaces, and also shows sev-
eral lines of growth near the margin. .

DEVELOPMENTAL VARIATIONS

No marked changes occur in the general form of the shell, other than
the gradual increase in the convexity of the dorsal valve and in the exten-
sion of the cardinal angles. The dorsal valve is usually quite flat in speci-
mens having a length of 1o mm. or less. The radiating lines increase in
number by interstitial additions, from the youngest form to maturity, and
the fine concentric striz appear on all the specimens, including the initial
individual in the series, where they are developed around the margins
of the valves.

The earliest phase of the hinge yet noticed, is found in a specimen

_—

SILURIAN BRACHIOPODA 25

having a length of 2.25 mm. The dorsal valve shows a foramen in the
cardinal area under the beak, margined by a slight thickening of the shell.
The ventral valve preserves a small perforate pedicle-tube at the apex,
extending about two-thirds of the distance down to the hinge, below which
is a triangular deltidial opening of the same width as the dorsal foramen.

A specimen 4 mm. in length (plate 2, fig. 19), shows a more advanced
development of the same parts. The dorsal callosity has nearly filled the
sinus under the beak and has a narrow groove in the center. The fissure
of the ventral valve has increased considerably in size and relative height,
showing narrow marginal plates, or defined areas which may be deltidial
plates. The pedicle-tube is still perforate, but has not increased in size
beyond the initial stage.

From this point to maturity, the hinge increases in width, the dorsal
callosity grows rapidly and nearly fills the fissure of the opposite valve.
The pedicle-tube is obscured and the perforation obsolete. The deltidial
plates, or lateral areas, are clearly defined, and have the form of narrow
scalene triangles.

No important variations have been noticed among the mature speci-
mens. Occasionally an individual diverges from the normal form by having
mucronate cardinal angles, or a senile specimen shows strong imbricating
varices of growth, but, as a whole, the form and surface ornaments in this
species are very uniform.

STROPHONELLA striATA, Hall, 1843
PLATE III, Fics. 1-8

Strophodonta striata, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 152, pl. 23,
figs. 1-6. 1879.
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 290, pl. 23, figs. 1-6. 1882.

The present form is one of the most delicate and fragile species of
brachiopoda at Waldron. Individuals are not of rare occurrence, but the

majority of them are more or less broken. The upper portion of the shell,
of,

26 MEMOIRS OF THE STATE MUSEUM

or that along the hinge, being thicker and stronger than the remainder, is
more often preserved, and the series is only complete in the representation
of this portion, although there are several small specimens which are suffi-
ciently entire to show the early form of the shell.

As in the other species which, in their mature proportions, depart from
the type of structure in the group, the incipient shell is found to revert to
the primitive form. The full-grown examples of this species are concavo-
convex, the concave valve being the ventral, while in the young, the ventral
valve is the more convex. This change in the relative convexity of the
valves does not begin until the individuals are about half-grown, and is pro-
duced by the gradual deflection of the margin with the increase in the size
of the shell. :

The development of the features of the hinge is very characteristic,
and, as in the other strophomenoid forms, is of primary interest. Both the
dorsal callosity and pedicle-tube continue to increase in size with the
growth of the shell, from the incipient form to maturity.

.

SPECIFIC CHARACTERS

Mature Form (plate 3, figs. 2, 2a, 8). Shell semi-elliptical, wider
than long, the greatest length being along the hinge. The body cavity is
very shallow, and the shell has a concavo-convex form.

Dorsal valve flat in the upper part, moderately convex in front. Ven-
tral valve slightly convex on the umbo, and concave over the remainder of
the valve. Hinge-area formed by both valves. Ventral area the wider,
carrying in the center a small conical pedicle-sheath which is usually min-
utely perforate at the apex. Dorsal area linear, with a callosity in the
middle, under the pedicle-tube of the opposite valve.

Test thin, surface ornamented by about fifty alternating radii, with
three or four fine filiform striae in each interspace; also crossed by fine
irregular striz of growth.

SILURIAN BRACHIOPODA 27

Two specimens measure respectively, 19.5 mm. and 14 mm, in length,
and 23 mm. and 16 mm. in width, at the hinge.

Incipient Shell (plate 3, figs. 1, 1a, 3). The form is nearly plano-
convex. Dorsal valve convex on the umbo, flat below. Ventral valve moder-
ately convex, with a prominent pointed beak. Hinge narrow, with a small
cylindrical perforated pedicle-tube in the center of the ventral area, and a
small callosity in the dorsal area. In the smallest specimen observed, the
surface is marked by eleven radii on the ventral valve, but is otherwise
apparently smooth. Length 2.25 mm.; width in the center, 3 mm.

DEVELOPMENTAL CHANGES

On account of the imperfection of the material, it is impossible to trace
any minor changes in the outline of the valves, and the specimens indicate
that no considerable transformation took place. The modifications in the
convexity of the valves is of more importance in this species, and can
be readily observed. In the young individuals, up to about one-third full
size, the ventral valve is slightly convex and the dorsal valve nearly flat.
Further growth of the shell changes these relations, by the gradual deflec-
tion of the margin, until the general form of the ventral valve is concave
and the dorsal valve is convex.

The radii appear very early in the growth of the shell, the smallest
individual having eleven on the ventral valve, the majority of which extend
to the umbo. They probably first appeared in pairs and are found to
increase in number afterward by simple intercalation.

The hinge-area is developed upon both valves in all stages of growth,
although in the early stages the cardinal area of the dorsal valve is very
narrow, but gradually increases, until at maturity, it is nearly equal to the
area of the ventral valve.

The pedicle-tube is at first cylindrical and short. Advancing in the
series, it is found to become conical from growth, and from the widening of
the fissure, until, in full grown specimens, it is wider than high. A careful

28 MEMOIRS OF THE STATE MUSEUM

examination reveals the perforation in all stages of the development of the
shell. It is, however, very minute, and it is not probable that the extremely
small peduncle could have performed its full function. Indeed, it may
be surmised that in none of the three strophomenoid species here described
was the fleshy arm sufficiently strong in mature individuals to serve as a
secure support to the shell. In the embryonic forms it was a more impor-
tant organ.

The hinge of the young shell illustrated in figure 4, plate 3, shows
an excessively elongate, cylindrical pedicle-tube, of which more than one-
half the length is projected above the beak. It must be considered as a
supra-calcification about the peduncle, and apparently indicates a more
complete functional extension.

The grooved dorsal callosity appears in the beginning of the series, and
gradually increases in size, and detrudes so that the groove shows on the
exterior, but just before maturity it is either filled, or introverted into
the deltidial cavity.

Tue FEATURES Of the hinge, fissure and callosity, in the Strophomenide,
and their embryological development, seem to be peculiar to the group.
They are of special interest both on this account, and also because the
family has no living congeners. Although the separate characters have
been presented in detail in each of the preceding descriptions, a brief review
of the hinge characters is here given, showing more clearly their intimate
relationships.

In the three species, Strophomena rhomboidalis, Strophonella striata and
Streptorhynchus subplanum, the initial form of the hinge is the same. Each
shows a slender callosity under the beak of the dorsal valve, and a perfo-
rate pedicle-sheath in the ventral valve, which does not entirely close the
deltidial opening. From this initial stage, development proceeds in a dif-
ferent manner for each of the three species. Strophomena rhomboidalis and
Strophonella striata develop in a parallel series until the individuals are

a9

SILURIAN BRACHIOPODA 29

about one-third grown in the first species, and two-thirds full size in the
second. That is, the dorsal callosity and pedicle-sheath each increase
uniformly in size up to these periods. Beyond this the divergence is rapid
and marked. In Strophomena rhomboidalzs, the grooved callosity increases
in size, so as to nearly fill the broad fissure in the ventral valve, while the
pedicle-sheath ceases growth, is atrophied and lost, although, in many
cases, the perforation persists. Strophonella striata continues its hinge de-
velopment without a change, except that, at full maturity, the groove on
the callosity becomes introverted into the pedicle-sheath.

The third mode of development is exhibited by Streptorhynchus
subplanum, in which the pedicle-sheath does not increase beyond its initial
size, while the dorsal callosity develops up to the maturity of the shell,
and, as in Strophonella striata, the groove is on the inner side.

The function of this groove in the callosity of the dorsal valve in the
strophomenoids has not been satisfactorily determined, its existence having
sometimes been considered as evidence of the perforation of this valve.*
In all young shells, it is evident that the passage of the pedicle is not
through this groove in the dorsal callosity, but through the apex of the
ventral valve by means of the channel which has been here termed the
pedicle-tube or sheath. In growth-stages where there can be no ques-
tion of the functional activity of this sheath, the dorsal callosity is already
grooved or sinuate. It might be surmised that the purpose of the groove
was to avoid compressing the pedicle when the valves were open, and this ~
it may have been to some extent; but the evidence furnished by both
recent and fossil species indicates that the valves of the articulate brachio-
pods could be opened only a very slight degree. The groove persists in
species after the true pedicle-perforation in the ventral valve is closed and
functionally useless. Its origin appears to be due to the organic deposition
about the bases of the two interior cardinal processes, the interstitial area
of slower deposition being represented by a fissure, groove or sinus.

*Eleventh Rept. State Geologist Indiana, pp. 288, 289. 1882.

30 MEMOIRS OF THE STATE MUSEUM

STREPTIS WALDRONENSIS, Miller and Dyer, 1878

‘

Piate III, Fics. 9, 10

Spirifera? waldronensis, Miller and Dyer. Contributions to Palzontology, Jour. Cinti. Soc. Nat.
Hist., April, 1878.

Triplesia putillus, Hall. Trans. Alb. Inst., vol. x, Abstract, p. 16. 1879.

—— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 298, pl. 27, figs. 19-22. 1882.

This species is among the rarest of the Waldron brachiopoda
and it is impossible to present a series representing the variety and progress
of development, as in some of the more common forms. There were but
two specimens, both adults, discovered in the State Collection at the time
of the publication of the “ Descriptions of New Species of Fossils from
the Niagara Formation at Waldron, Indiana.”* Fortunately, we have
more recently detected a young individual of about one-fourth the normal
adult size, which offers some interesting details in its form and characters.

The asymmetry of the shell is manifest even at this early stage of
growth (although the median fold is not developed), and is evinced by the
position of the beak of the ventral valve, and by the contour of the margins.
It is probable that in a still earlier phase of growth, the two valves are
symmetrical, or nearly so.

In the young individual under consideration (plate 3, figs. 9, 9a, 96),
the outline is nearly circular. The beak of the ventral valve is very much
elevated, projects beyond the cardinal line and is directed toward the
- left side of the shell. The apex is truncated, and the opening is confluent
with the area below. %

The cardinal area is high, forming a large triangular fissure which
is apparently not closed by deltidial plates. The beak of the dorsal valve
is depressed, and limited by a slight furrow on each side. No lines of

growth are visible, but the surface is somewhat granulose, as in many small
shells of other species,

*James Hall. Read before the Albany Institute, March 18, 1879.

SILURIAN BRACHIOPODA 31

The principal differences to be noted in comparison with the adult
individuals are the subcircular outline of the shell, the depressed valves,
the absence of a median fold, and the large deltidial area.

EICHWALDIA RETICULATA, Hall, 1868
Prate III, Fics. 11-13

—— —, Hall. Twenty-eighth Ann, Rept. N. Y. State Mus. Nat. Hist., p. 169, pl. 26, figs. 50-54.
— pis “all. Eleventh Ann. Rept. State Geol. Indiana, p. 312, pl. 26, figs. 50-54. 1882.

Very few of the earlier growth stages of this species have been
observed and these show but comparatively little variation from the
features of the normal adult. On plate 3 is given a figure of the youngest
example found, which has a length and width of 3 mm., while the usual
adult is about 16x16 mm., varying in relative proportions with the increase
of stnile obesity. The change in outline during growth is from
subcircular to subtriangular, and in earlier stages, the ventral fold and
sinus are very ill-defined. The peculiar triangular exfoliation of the shell
on the umbo of the ventral valve is evidently a constant feature in every
stage of growth after the shell becomes attached. The nature of this
peculiarity was indicated by Briiincs in the original diagnosis of the genus
(Ann. Rept. Canadian Geol. Survey, 1857-58), and was demonstrated more
fully by Professor Hatt, in the Twentieth Report on the Condition of the
New York State Cabinet of Natural History (pp. 274-278, 1867). This
area is underlaid by an internal shelf or diaphragm attached along its lateral
margins, and having fully, or rather more than, the width of the median
sinus. Through the space thus left between the shell and the internal dia-
phragm, communication is afforded with the outside world. Mr. Joun Younc |
has called attention to the fact that in &. capewel/’, the margins of the ex-
ternal reticulated layer of the shell about the umbonal bare spot, are rough
and ragged, the superficial hexagonal cells being without finish along these
edges, suggesting therefrom, that the animal was attached to marine

32 MEMOIRS OF THE STATE MUSEUM

objects by the substance of the shell, and afterward broken away from
its attachment. (See Davipson, General Summary, pp. 355, 356.) It
is true that the anterior edge of this area may be rough and uneven,
but the lateral edges appear invariably straight and diverge at an essen-
tially constant angle. The latter represent the lines of attachment of
the internal plate to the interior of the valve, and if the shell has been
broken in detachment from foreign bodies, the fracture in these direc-
tions has been guided by these lines, but on the unsupported anterior
margin it has been rough and irregular. Upon the hinge-line of the
ventral valve, there exists no aperture for the protrusion of the pedicle ; by
the peculiar development of the articulating processes of both valves, the
entire cardinal margin is closed, and therefore the passage between the
internal plate and the surface of the valve may have been for the use of
this organ; or, it may be suggested, that as this space is rather too
narrow and explanate for such a purpose, Lzchwaldia may have been
attached by the substance of the shell, the internal shelf acting as a
support to the strain upon the umbo, and a protection to the animal in
case the shell were broken from its attachment.

ANASTROPHIA INTERNASCENS, Hall, 1879
PLATE III, Fics. 14-16
— —, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 168, pl. 26, figs. 41-
—— ee tal Ann. Rept. State Geol. Indiana, p. 311, pl. 26, figs. 41-49. 1882.

In tracing the development of this species, the principal feature to
be noticed is that the elemental shell conforms with the type of an ordi-
nary brachiopod, such as Rhyuchonella, that is, the dorsal valve, although
somewhat the more convex, is smaller than the opposite valve, while
in the mature state the dorsal valve is considerably larger and pro-
jects beyond the beak of the ventral valve. It is the development of this

SILURIAN BRACHIOPODA 33

character which constitutes the most conspicuous change in the shell in
its growth from the young to the fully mature condition.

SPECIFIC CHARACTERS
Mature Form (plate 3, figs. 15, 16, 162). Shell ventricose. Out-

line transversely subelliptical, sometimes nearly as long as wide.

Ventral valve convex, depressed in front, forming a more or less
defined sinus which carries four or five of the plications; beak short, acute ;
area short, broadly triangular, usuallysnot exposed.

Dorsal valve gibbous, with the central portion elevated, frequently
presenting a broad undefined median fold ; beak incurved under.the beak of
the opposite valve ; umbo prominent.

Surface marked by about fifteen strong, simple, elevated, rounded or
angular plications on the body of the shell, and smaller bifurcating plica-
tions on the latera. Occasionally intercalated plications are present on the
middle of the valves. The plications are crossed by fine arching strie of
growth, which are sometimes aggregated, forming conspicuous concentric
lines or varices of growth.

Mature shells measure from 11 to 17 mm. in length, and from 12 to 19
mm, in width. The depth of the conjoined valves varies from 9 to 12 mm,

Incipient Form (plate 3, figs. 14, 142). The smallest shell observed
has a length of 2 mm. and a width of 2.25 mm. The dorsal valve is slightly
more convex than the ventral and is a little shorter. Eight rounded plica-
tions are shown, five of which extend to the umbo of the valve. A short
plication is intercalated in the middle and there is also a short one on each
side of the valve. Ventral beak small and elevated, with a broad, triangular,

open area below.
DEVELOPMENTAL CHANGES

The series of specimens selected to represent the development of this
species contains fifteen normal individuals, varying from a length of 2 mm.
toa length of 17mm. The proportions of length and width remain nearly

constant throughout, the width being somewhat the greater.
5

34 MEMOIRS OF THE STATE MUSEUM

In the smallest specimen, the depth of both valves is less than one-half
the length of the shell. This relation gradually changes as the shell
becomes larger and more convex, until, in mature individuals, the depth is
equal to three-fourths or four-fifths the length of the shell, and in extremely
obese specimens this ratio is often exceeded.

The dorsal valve is more convex than the opposite valve, in all the
stages of growth which have been observed, although in the elemental shell,
the difference is scarcely perceptiblag while in the mature form it is a con-
spicuous feature. This valve is also shorter than the ventral in specimens
up to a length of about 7 mm. From 7 to 12 mm., both valves are of nearly
equal length. Further growth causes the umbo of the dorsal valve to pro-
trude beyond the beak of the opposite valve, and the beak is incurved and
penetrates the area. It seems evident that if we had the true initial shell,
the dorsal valve would be found not only smaller but less convex than the
opposite valve.

The fold begins to be apparent in individuals having a length of about
10 mm., and is expressed by the arching of the anterior margin. It does
not sufficiently develop to become a characteristic feature and is more or
less undefined, even in many full-grown specimens.

The plications increase both by bifurcation and interstitial addition.
The smallest number observed is eight, and this is gradually increased with
the growth of the shell, until there are about fifteen principal plications on
the body of the shell, and several smaller ones just below the cardinal
extremities. The concentric striae are not often preserved and the plica-
tions therefore form the only conspicuous character of the surface orna-
mentation.

The deltidial plates reach but a slight development, the triangular
opening becoming completely filled by the incurved beak of the dorsal

valve.

SILURIAN BRACHIOPODA 35

RHYNCHONELLA acinus, Hall, 1863
PiaTE IV, Fics. g-11

—— —., Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 306, pl. 26, figs. 7-11.

1879.
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 306, pl. 26, figs. 7-11. 1882.

Were Rhynchonella acinus a rare species, it might readily be confounded
with the variety of 2. zzdianenszs, which bears but a single plication in
the ventral sinus. It appears, however, to have been very prolific, and its
abundance serves to emphasize its specific independence. The liability to
confuse it with any of the associated species arises only among forms of
immature growth. Beginning with a shell which is apparently in the actual
initial stage, measuring 1.2x.8 mm., our series is very evenly consecutive
up to maturity, when the average dimensions are 8x6 mm.

SPECIFIC CHARACTERS

Mature Form (plate 4, figs. 11-114). Shell small, longitudinally
ovate, subattenuate toward the beak, and truncate in front. Cardinal
margins long and rapidly sloping, extending more than half-way across
the shell; sides flattened, slightly excavate. Valves subequally convex.

Ventral valve full and rotund on the umbonal region, flattened at
about the middle, thenceforward sinuate; beak ihcurved, but not pro-
cumbent ; foramen generally concealed, or when slightly exposed, elongate
or subtriangular.

Dorsal valve more flattened in the umbonal region and in the middle,
whence a low fold proceeds to the margin.

Surface marked by low rounded plications. The ventral sinus
bears a single plication which is generally faint, often nearly obsolete. On
each side of the sinus are four plications, ‘those abutting on the cardinal
margins being indistinct. On the dorsal valve, the low, flattened fold bears
two plications which are the strongest upon the shell; these are accom-
panied by three plications on each latus, making the whole number on this

-

36 MEMOIRS OF THE STATE MUSEUM

valve eight. No concentric growth-lines are apparent. Average dimen-
sions 8x6 mm.

Variations from the normal adult. Two plications sometimes occur
in the sinus, and in such cases they are each stronger than the single sinal
plication in the normal adult. The addition of the plication to the sinus
increases the number in the fold to three, and the total number of plica-
tions on the shell by two.

Initial shell (Compare plate 4, figs. 9, 9a, 94.) Two individuals,
one measuring 1.2x8 mm., the other 1.4x9 mm., apparently indicate the ini-
tial stages in the growth of this shell. Neither of these examples has
served well for illustration, on account of the lack of well-defined details,
but they may be described as follows :—Attenuate, subspatulate. Ventral
valve with erect, straight beak; cardinal area high, convex, with a promi-
nent dorsum. Jorsal valve flattened or slightly sinuate. Features of
the cardinal area not discernible; from analogy, the foramen would be
triangular and unobstructed. In figures 9-94, which show a secondary stage
of growth in the shell, the portion included within the first growth-line will
represent very well the characters of the primitive shell.

GENERAL DEVELOPMENTAL CHARACTERS
The gradual incurvature of the beak and consequent concealment of
the ventral foramen may be assumed from the foregoing. It harmonizes
with the associated species of the same genus in the slight variation in the
form and proportions of the foramen in consecutive stages of growth, as
well as in the reversal of the embryonic fold and sinus to the mature sinus
and fold. The plications of the latera seem to appear simultaneously after
the first varix, as shown in the figures referred to, and their number does
not change materially until maturity. The embyronic sulcus on the dorsal
valve, correlate with the ventral dorsum in the primitive stage, is continued
at maturity into the median sulcus separating the two plications of the
dorsal fold.

SILURIAN BRACHIOPODA , | 37

RHYNCHONELLA NEGLECTA, Hall, 1852
: PLATE IV, Fics. 3, 6-8
— —, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 162, pl. 26, figs. 1-6.
— eee, Eleventh Ann. Rept. State Geol. Indiana, p. 305, pl. 26, figs. 1-6; pl. 27, fig.
3. 1882.

For a species so abundant as this in the Waldron fauna, the diagnos-
tic features are.retained with unusual persistence within very narrow limi-
tations. Unlike its associate, R. zxdzanenszs, which it almost equals in
numerical representation, there are -no well-established and perduring
variations from the normal adult form,-and our observations are therefore

limited to an essentially unvarying phase.

SPECIFIC CHARACTERS

Mature Form (plate 4, figs. 8, 8a). Shell small, transversely sub-
ovate; umbo scarcely prominent. Cardinal slopes long and _ flattened,
rounding to the anterior margin which is nearly straight.

Ventral valve with the umbo elevated and slightly incurved at the tip,
overhanging an elongate subtriangular foramen. Umbonal region slightly
convex, the convexity extending for one-third the length of the shell ; thence
forward the shell is rapidly depressed medially to form a deep sinus, which
makes a high quadrangular extension on the margin; lateral portions
depressed.

Dorsal valve with the umbo low and inconspicuous; apex concealed
within the foramen of the opposite valve. The shell becomes rapidly ele-
vated medially to form the fold, the latera being full and convex.

Surface covered with regular, sharp and prominent plications, which do
not vary in number at normal maturity, and which, in the growth of the
shell, are increased only from the cardinal margins. Of these plications,
the fold bears four, the sinus, therefore, three, and each of the latera, five,
those nearest the cardinal margins being obscure. This makes in all for the

38 MEMOIRS OF THE STATE MUSEUM

.
ventral valve, thirteen, and for the dorsal, fourteen plications. Dimensions
of an average example, length, width and depth, 9, 8 and 5 mm.

A single individual presents the only important abnormality noticed,
viz.: a failure to produce the requisite plications upon the latera, the dorsal
valve bearing but five, and the ventral, six. Of these, three are on the
fold, two in the sinus. It is interesting to notice that in the umbonal
region the normal number of plications had been formed in their
regular arrangement; their disappearance on the latera and irregular
disposition in fold and sinus took place abruptly upon the completion of a
growth-line 2 mm. from the apex. This is a marked instance of reversion
after the assumption of certain adult features.

Incipient Form (plate 4, figs. 6, 6a). The example with which
our series opens measures .75x.5 mm. It is elongate subtriangular, with
the ventral beak elevated and erect, the cardinal margins sloping for two-
thirds the length of the shell; foramen triangular, slightly, if at all, en-
croaching upon the apex, without deltidial plates, margins thin; dorsal
beak rounded, inconspicuous. At one-third the distance from the apex to
the anterior margin, fine thread-like plications appear, four upon the dorsal,
and three (five?) upon the ventral valve. The median sulcus on the
dorsal valve is broader and deeper than any other, forming the embryonal
sinus, and is accompanied by a correlatively strong plication on the oppo-
site shell.

DEVELOPMENTAL VARIATIONS

General Form and Outline. The form of the shell varies from
dimensions in which the length is one-quarter greater than the width, to
those of maturity when the width is slightly greater than the length, The
depressed, subspatulate embryo eventually becomes convex and deep.
The embryonal sinus and fold on the dorsal and ventral valves respectively,
are never so prominent as in A. zwdianenszs, and soon become lost, the

SILURIAN BRACHIOPODA 39

former in a sulcus, and the latter in one or more sulci upon the reversed
fold and sinus of maturity.

Beak and Foramen. The erect and acute beak of the elementary
stages of growth, becomes, at maturity, but slightly iacurved, and never
procumbent on the dorsal umbo. The foramen, at the outset triangular,
subsequently has its margins thickened, and develops small and obscure
deltidial plates at its base, which at maturity leave the foramen elon-
gate and not circular. In respect to these features, the development of
the species is identical with that of A. zxdzanensis.

Plications. In the first observed stadium, only the umbonal area is
smooth, and from the analogy of A. zxdanensis, it would appear that the
initial growth-stage yet fails us. Ata size of 2.5x2 mm., the number of
plications has increased from four to ten on the dorsal, and from three
(five?) to eleven on the ventral valve, and this, added to a pair of ex-
tremely obscure plications near the cardinal margins, is the normal number
for maturity.

RHYNCHONELLA WHITH, Hall, 1863
PLATE IV, Fics. ft, 2, 4, 5

—— —, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 164, pl. 26, figs. 23-33.
1879.

— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 307, pl. 26, figs. 23-33. 1882.

Like Rhynchonella neglecta, this species is subject to very slight vari-
ations at maturity, and its specific expression is well marked, but a certain
embarrassment attends the first endeavor to separate the immature individu-
als from those of allied species. This, however, disappears with a careful eye
properly estimating the essential characters of the species. The earliest
stage of growth we have found measures 2.75 mm. in length by 2 mm. in
breadth, and from this size upward to that of 13x13 mm. all variations are
present.

40 MEMOIRS OF THE STATE MUSEUM

SPECIFIC CHARACTERS

Mature Form (plate 4, figs. 2, 2a, 26). _ Shell transversely sub-
elliptical ; length and width about equal.

Ventral valve shallow ; beak high, acute, somewhat attenuate, with the
apex slightly incurved, but not concealing the triangular unclosed foramen
which reaches entirely across the cardinal area. At its apex the foramen
encroaches slightly upon the umbo, and is narrowed somewhat toward the
base by the imperfectly developed deltidial plates. A median depression
makes its appearance at about one-third the distance from the umbo to the
anterior margin, and soon develops into a deep sinus with sharply sloping
sides.

Dorsal valve deeper and more gibbous; beak inconspicuous, and
incurved beneath the ventral foramen. A strong median fold corre-
sponds in development with the median sinus of the opposite valve.

Surface marked by strong, simple, subangular plications, invariably
two upon the fold and one in the sinus, with six on each of the latera,
making thirteen on the ventral, and fourteen on the dorsal valve. Of these,
the plications near the cardinal margin are low and incipient, but the full
number becomes permanent early in the history of the individual. Faint
concentric growth-lines are sometimes visible. Dimensions of average
adult 11x11 mm.

Abnormalities at maturity. The variations from the normal ma-
ture form are, as far as observed, wholly due to continued internal growth
after individual maturity has been attained, and this is to be regarded as
the concomitant evidence of senescence. There may be either a margznal
thickening, which gives the shell a truncate appearance, or a general z¢ernal
thickening, making the shell unusually gibbous, and forcing the ventral beak
over upon the dorsal umbo.

Incipient Form (plate 4, figs. 1, 1). The youngest individual
observed measures 2.75x2 mm. ; outline subovate, valves regularly rounded,

SILURIAN BRACHIOPODA | 41

the ventral being the more convex. Ventral valve with an erect, straight
beak; apex acute, cardinal margins sloping rapidly forward, and slightly
excavate. Foramen simple, triangular, free from deltidial plates, encroach-
ing at its apex slightly upon the umbo; foraminal margins somewhat thick-
ened, Dorsal beak erect but inconspicuous, full and rounded. Dorsal
valve depressed anteriorly along the median line, this depression correspond-
ing with the broad and low dorsum of the opposite valve. Surface of each
valve marked by eight single, rounded plications, which extend two-thirds
the distance from the anterior margin of the beak, leaving the circumbonal
area smooth,
DEVELOPMENTAL VARIATIONS

General Form and Outline. As growth advances, the development
is more rapid transversely than longitudinally, and, consequently, the subo-
vate incipient shell becomes, at maturity, broadly transverse. The prominent
dorsum of the ventral valve in the embryo, is manifest at maturity only in
the rounded and prominent beak, and the embryonal sinus in the dorsal
valve becomes so thoroughly obsolete at maturity as to be unnoticeable.
In stages of development between the dimensions 3.5x3 mm. and 6x5.5 mm.,
the ventral valve still retains a slightly greater convexity, but the anterior
margin is entire. .

Beak and Foramen. The erect and straight beak of the incipient
shell becomes slightly incurved toward maturity, but the cardinal area re-
mains high, exposing the triangular foramen at all stages of growth. Del-
tidial plates make their appearance early, but never develop sufficiently to
meet and inclose the pedicle-aperture, a feature indicative of arrested de-
velopment, and equally true of the other members of the genus here
discussed. .

Plications. The fact that the eight plications on each valve of the
incipient shell do not reach the umbones, indicates that the initial shell may

have been smooth, as we have shown it to be in 2. zxdzanensis. The
6

42 MEMOIRS OF THE STATE MUSEUM

subsequent addition of plications takes place slowly and from the cardinal
margins.

RHYNCHONELLA INDIANENSIS, Hall, 1863
Pate III, Fics. 17-28

—— ——, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 163, pl. 26, figs. 12-22.

1879.
————, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 306, pl. 26, figs. 12-22; pl. 27, figs.
4-6. 1882.

Rhynchonella tndianensis is, beyond a doubt, the most prolific species in
the rich fauna of the Waldron beds, and by virtue of this fact, we have been
enabled to ascertain the developmental phases through illustrative series of
exceptional completeness. It is noteworthy that the mature shell of this
Rhynchonella presents variations from the adult type, which are so great that
in a certain sense they might be regarded as passing the limitations of
specific identity ; however, the general form and expression of the shell are
characteristic, so that, in spite of these variations, no confusion with allied
species of the same fauna can arise, nor need there be any hesitation to
assign to the different forms a varietal significance only. Probably ten
thousand individuals of this species have passed under our observation, and
of this large number, fully one-half have been immature forms.

SPECIFIC CHARACTERS

Normal Mature Form, containing two plications in the sinus of the
ventral valve (plate 3, fig. 21). Shell subtriangular or broadly
ovate ; length nearly equal to, sometimes slightly exceeding the width,
Umbo prominent, subacute; cardinal slopes extending one-half the length
of the shell, and flattened.

Ventral valve depressed convex, rounded at the beak; apex pointed
and slightly incurved, exposing beneath it the elongate, narrow foramen
and the inconspicuous deltidial plates. Dorsum for the first one-third the
length of the shell rounded, thence, anteriorly, gradually becoming de-

SILURIAN BRACHIOPODA 43

pressed. The sinus thus formed bears two strong, rounded plications which
are of later origin than the pair which forms its lateral boundaries, The
latera bear each three plications with traces of a fourth, making eight (ten)
on the entire surface of the valve.

Dorsal valve somewhat deeper than the ventral, flattened above,
depressed near the beak along the median line (embryonal sinus), thence
forward, becoming gradually elevated into a fold which bears three strong
rounded plications. Four similar plications are discernible on each of the
latera, making in all eleven plications on the entire valve. Umbo incon-
spicuous, apex concealed within the foramen of the opposite valve, Con-
centric growth-lines obscure, or absent. Average dimensions 12x12 mm.

These are assumed as the normal characters of adult growth on account
of the great predominance of specimens bearing ¢wo plications in the ven-
traf sinus.

Variations from the Normal. A. Forms with one plication in the
ventral sinus. This variation does not attain quite the size of the average
normal adult, but retains the same proportion of length and breadth (size
1ox1o mm.). ‘The surface bears ten plications on the dorsal and nine on the
ventral valve. In this form, the embryonal sinus, visible on the earlier por-
tion of the dorsal valve, is distinctly continuous with the strong sulcus
separating the two plications on the fold in the later and marginal portions
of the valve. This variation is not of uncommon occurrence, and immature
individuals in various stages of development prove that it is a well-estab-
lished genetic difference, and not merely an occasional monstrosity.

B. Forms with three plications in the ventral sinus. The size and pro-
portions of the normal are retained in this variety, but the shell bears
usually three, sometimes four plications on each of the latera, making ten
(twelve) plications for the dorsal, and nine (eleven) for the ventral valve.
This form is of comparatively rare occurrence, and is not often noticed in an
immature stage of growth.

44 MEMOIRS OF THE STATE MUSEUM

C. Forms with four plications in the ventral sinus. This variation is met
with very infrequently, but two individuals having been obtained. While
agreeing in size with the normal adult, the crowding of the sinus with pli-
cations tends to obliterate both it and the fold upon the opposite valve.
Both individuals show the interesting fact that upon the dorsal valve where
the fold bears five plications, i. e., four sulci, the embryonal sinus is contin-
uous with the ¢/zrd of these sulci, in-one instance numbering from the right,
in the other, numbering from the left. Of the five plications which are thus
separated into groups of three and two, it is noticeable that the outer
member of the group of three is both less elevated and shorter than any~
other upon the fold.

Monstrous Forms. The sole evidence of monstrous growth that has
been observed is an asymmetrical development of the plications upon the
dorsal fold. Examples bearing three plications upon the fold, in rare
instances have one of the plications very large and two quite small, making
one broad and one narrow sulcus upon the fold. The phenomenon may be
due to the strongly developed tendency of the embryonal sinus to maintain

its continuity with a median sulcus even at the expense of the symmetry
of the shell. ;

Initial Shell (plate 3, figs. 17, 17@). The initial shell in our series
of Rhynchonella indianensis measures .65 mm. in length by .54 mm. in width,
It is broadly ovate or subpyriform in aspect, convex posteriorly, and
depressed toward the anterior margin. Ventral valve with the umbo
prominent, the beak elevated and erect, with the apex rounded; cardinal
margins rapidly sloping. Foramen subtriangular, apical portion broader
than usual in the incipient stages of plicate shells ; margins not thickened ;
deltidial plates absent. Dorsal valve with a rqunded, inconspicuous beak.
Surface of both valves quite smooth. A median depression is noticeable
on the dorsal valve near the anterior margin, making this margin sinuate.
This embryo is the smallest that has been found for any of the series

SILURIAN BRACHIOPODA 45

of Rhynchonellas, and not only on account of its minuteness, but also from
the entire absence of plications on its surface and from the elementary
character of the cardinal area, we are inclined to regard it as the actual ele-
mental or initial shell.

. DEVELOPMENTAL VARIATIONS

General Form and Outline. The adult variations from the normal,
noticed above, seem to be in most instances, and probably would prove to be
in all, preceded by well-defined embryonic series leading up to them.
This must be the case, as the character of these variations, i. e.,
variation in the number of plications on the median portions of the
shell, is such that they cannot be assumed after the attainment of the adult
condition, as is possible’in certain other forms of variation. But it is not to
be assumed that the conformation of the embryo which eventually pro-
duces any of these results, manifests them in the earliest stages of the
growth of the shell, rather, that the shells, under whatsoever variations at
maturity, all have the same unspecialized starting point. Hence, the fact
that some of these variations have not shown a complete series of immature
' stages must be due to the insufficiency of our material, rich as it has been.

Limiting our considerations now to the normal form with ¢éwo plications
in the ventral sinus, we notice that the initial shell is smooth, and obcordate
in outline, with beak erect, while the mature shell is strongly plicate, strongly
ovatewand--with the beak sharply incurved. The transition. from one
extreme to the other is through stages of growth between the -limits
-65x.54 mm. (initial) and 12x12 mm. (average adult), In growth-stages
below 7x5 mm. dimensions, the shell is very depressed-convex, the dorsal
valve up to about this point, retaining a low, broad, median depression,
accompanied by a similarly low and broad median elevation on the opposite
valve. It is not always possible to determine with accuracy how many
plications are carried by this embryonic fold and sinus, on account of its
not being well limited, but their eventual reversion, in the adult shell, into

46 MEMOIRS OF THE STATE MUSEUM

sinus and fold, respectively, marks the feature as an interesting one, to
which we call attention more at length in the description of the species
Rhynchotreta cuneata and Atrypa reticularis. Rare instances occur of
individuals assuming all the characters of maturity before attaining a
length of 6 mm., and from this point upward to the normal size for adult
growth, mature dwarfs are frequently found.

Beak. In the initial shell, the beak of the dorsal valve is rounded and
inconspicuous, and so remains in all stages of growth. In the opposite
valve, the beak is at first high, erect but not acute, the cardinal margins
sloping abruptly, and with increasing age the beak becomes fuller, more and
more incurved at the apex, but is never closely procumbent upon the dorsal
umbo, as is the case in maturity with most of the plicate species here
described,

Foramen. At the outset the pedicle-aperture is narrowly subtrian-
gular, reaching to and encroaching upon the apex, free of deltidial plates
and with the lateral margins unthickened, i. e., elemental in every respect.
In the second stage. of growth (after the appearance of plications on the
surface, dimensions 1.5x1.1 mm.), the apertural margins have become
thickened, and directly thereafter, the deltidial plates begin to develop,
gradually narrowing the aperture at the base. The symphysis of these
plates with the valve is marked by distinctly elevated lines. In maturity,
the deltidial plates have developed sufficiently to completely close the lower
part of the aperture, coming together behind the beak of the dorsal valve,
and giving to the foramen an elliptical outline constricted toward the apex,
where it encroaches upon the umbo, The fact that the development of the
foramen is thus interrupted before it reaches the circular outline normal to
the adult of most paleozoic species, indicates an embryonic character in
the adult, and, therefore, a subordinate taxonomic position for the species.

Plications. These appear only after the first stage of growth is
passed and after the first growth-line has been formed. As in Reézta evax,

SILURIAN BRACHIOPODA 47

they appear over the entire surface of the shell below the growth-line all at _.
once, and from this stage onward to maturity, no increase is made in the
number, except by intercalation along the margin of the fold and sinus.

RHYNCHOTRETA CUNEATA, Dalman, 1827

var. AMERICANA, Hall, 1879
Prater IV, Fics. 12-22

—— —— — Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 167, pl. 25, figs.

29-38. 1879.
—— —— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 310, pl. 25, figs. 29-38. 1882.

The individuals of this species do not so readily separate into three
groups of long, normal, and broad forms, as do those of Rhynchonella neg-
lecta, Retzia evax, Meristina nitida, and others. This seems to be due to the
uniformity in the number of plications, and also in the number carried on
the fold and sinus. The long and broad varieties do exist, however, but are
of such infrequency as to suggest that they are not genetic variations from
the typical form occurring in this locality.

The specimens from the Wenlock shales of Dudley, show a consider-
able variation from their American congeners, in having more numerous
plications, of which a greater number is raised on the dorsal fold and
depressed in the ventral sinus. In other respects, it is believed that the
description here given, of the development of the shell, will apply to the
British form.

Rihynchotreta cuneata, although considered as abundant in the mature
state, does not approach in the number of young specimens, RAyuchonella
; whitit, R. neglecta, R. indianensis, Retzia evax, Meristina nitida, Spirifer

crispus, Sp. crispus, var simplex, Atrypa reticularis, Orthis hybrida, and
O. elegantula, The entire numberof young individuals examined is about
one hundred and fifty, ranging in size from 8 mm. to 1.5 mm. in length. -
The mature forms average about 17 mm. in length.

48 MEMOIRS OF THE STATE MUSEUM

In the several series selected from the material at hand, it is evident
that the shell assumed the characters and form of maturity when reaching
a length of about 1omm. At this period of growth, the fold of the dorsal
valve becomes elevated, and the sinus of the ventral valve. depressed
(fig. 15, plate 4). Previous to this stage, the dorsal valve is depressed —
and transversely concave, and the plications of the opposite valve are raised
along the median line of the shell.

SPECIFIC CHARACTERS

Mature Form (plate 4, figs. 14, 14a, 148, 22, 222). Shell triangular,
cuneiform, widest across the pallial region. Length equal to about twice
the depth of the valves. Beaks compressed laterally, attenuate and pointed.

Ventral valve moderately convex, subangular along the latera, marked
by a deep sinus, which commences near the middle of the length, and
becomes very marked in front, depressing three plications, of which the
middle one is detruded more than the others.

Dorsal valve convex, gibbous in the posterior part, with the latera
elevated and subangular ; marked in front by a prominent fold which begins
near the beak as a depression carrying four plications, of which the two
central ones are usually much more elevated than the other pair.

Area high, closed by two triangular deltidial plates. Perforation of
the ventral beak ovate, truncating the apex, and limited below by the del-
tidial plates.

Surface marked by from eight to ten strong, angular plications, which
are crossed by very fine, regular, sharp, concentric striae. Mature specimens
usually measure from 10 to 17 mm. in length.

Incipient Form (plate 4, figs. 12, 12a). The youngest shell detected
has a length of 1.5 mm., is flattened, and nearly circular in outline. The
dorsal valve is depressed in the middle, and carries four plications. The
beak of the ventral valve is broadly triangular, exsert,-and elevated, with a

triangular, open area without deltidial plates.

SILURIAN BRACHIOPODA 49

DEVELOPMENTAL CHANGES

Contour. In the earliest stages yet noticed, the shell is nearly cir-
cular. Ata length of 2 mm., it is broadly oval, and at 2.5 mm., it is ovate.
The beak in the next advanced stage is more elongate, and when the length
of 4 mm. is reached, the shell has a decidedly triangular or cuneate form,
which becomes more pronounced up to maturity. All the young and
adolescent shells are depressed, the characteristic fullness of the valves not
being developed until after the assumption of the features of maturity,

and when the shell approaches its normal size.

Fold and Sinus. The smallest individual shows a slight depression
in the dorsal valve, co-existing with the plications, beginning about one-fifth
the length of the shell in front of the beak, widening rapidly, and becom-
ing more defined upon approaching the margin. The latera are nearly flat.
The depression, or sinus, becomes more pronounced with the advance in
growth, until a length of 4.5 mm. is attained. After this period, the four
bottom plications gradually elevate, the sinus grows shallower, and the front
margin of the conjoined valves becomes nearly straight. Upon reaching
a length of 9 mm., the two central plications are sufficiently elevated to
define the fold, which is hereafter the principal feature of the dorsal
valve. The development from this point to full-grown individuals is prin-
cipally directed to reaching a maximum prominence in the fold, and increas-
ing the shell by increment on the lateral margins of the valves.

The development of a sinus in the dorsal valve, its subsequent oblit-
eration, and the final elevation of the plications into a strong median fold,
are shown in figures 15, z—zo of plate 4, in which the undulating lines
represent the anterior junction of the valves.

Beak. The apex of the dorsal valve is strong and pointed, and is
visible in all specimens up to a length of about 14 mm. After this stage, the
shell becomes obese, and the consequent greater inclination of the beak, forces

it into the foraminal cavity, where it becomes hidden by the deltidial plates.
7

50 MEMOIRS OF THE STATE MUSEUM

The ventral valve is uniformly convex in all incipient specimens. The
sinus develops at the same period, and in conformity with the fold of the
opposite valve. The beak of the initial shell is broadly triangular, perfo-
rate at the apex, and directed outwards. It gradually becomes narrower and
less oblique with advancing growth, and lies in the axis of the shell in full-
grown specimens. The initial perforation is a small truncation of the beak,
confluent with the open area below. (.See figure 16, plate 4.)

Surface ornaments. The prevailing number of plications is eight, al-
though it varies from seven to ten in some specimens. The entire number
appears at an early period of growth, and in this respect, the species offers
a marked difference from some of the forms of RAynchonella already con-
sidered, in which the plications increase by pairs. In a specimen 1.5 mm.
long, they first appear at about one-fifth the length of the shell from the
beak. Four plications are included in the depression of the dorsal valve in
the incipient stages, and the two central ones finally become elevated,
forming the fold in the full-grown shell. Upon approaching maturity, three
of the plications in the ventral valve are depressed, the middle one ulti-
mately much more than the others, forming the single strong plication at
the bottom of the sinus.

No concentric striae are shown on the initial shell of our series. Ina
specimen 3 mm. in length, these begin to develop over the outer third of
the surface, as shown in figure 13, plate 4.

Cardinal area. The foramen is at first a broad triangular opening,
wider than high, with sharp margins, and truncating the beak of the ventral
valve. The lateral margins are thickened ina specimen 3 mm. in length
(plate 4, fig. 13), and the height and width of the area are equal. These pro-
portions of height and width are preserved to maturity, although in some
specimens the area is higher than wide. No deltidial plates have as yet
appeared, but in the next stage, including individuals having a length of
4.5 mm., there are two narrow deltidial plates developed from the sides of

SILURIAN BRACHIOPODA 51

the foramen (plate 4, fig. 18). A specimen 5 mm. in length shows the still
further increase in the size of these plates, although they do not come in
contact, but leave an oval opening extending from the ventral beak down
to the beak of the dorsal valve (plate 4, fig. 19). The increase in the
growth of the deltidial plates along their inner margins brings them in con-
tact under the dorsal beak, in specimens having a length of about 7 mm.
(plate 4, fig. 20). Further growth truncates their inger angles, thus short-
ening the deltidial opening. In individuals about 12 mam. long (fig. 21),
the opening extends but little more than half the length of the area, and
the lower margin of the opening is thickened and slightly deflected. Fully
matured forms, having a length of from 15 to 17 mm., have a perforation
less than one-half the height of the area, which truncates the beak more
strongly than in younger shells, and the deltidial plates show a defined
thickened area below the perforation, often extending to the dorsal beak
(plate 4, fig. 22).

Variations. As already stated, the elongate and broadly flabellate
shells appear to be, in this species, neither common nor genetic variations.
Among the extraordinary developments are specimens with duplicate pli-
cations in the sinus, and one showing but seven plications on the shell.
Another individual has the initial shell strongly defined by a varix of
growth, and shows on this portion ten plications, but in the subsequent
growth, only eight plications are continued, these alternating at the varix
with those of the embryonic shell.

ATRYPA RETICULARIS, Linnzus, 1767
Prate VI, Fics. 12-20

— -—, Hall. Twenty-fourth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 162. 1879.
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 304. 1882.

The abundance of this well-known species at Waldron has afforded the
means of studying its developmental stages with very satisfactory results.
All the individuals, from the earliest observed stage upward, agree in con-

52 MEMOIRS OF THE STATE MUSEUM

tour, there being no such variation in this respect as has been noticed in
some other species (e. g. Retzta evax, Meristina nitida) in which appear
deviations from the normal, producing a long type and a broad type. The
youngest individual detected has a length of 2.25 mm. and a width of
2mm., though this may not be regarded as the initial shell on account of
the presence of partially developed deltidial plates. From this stage of
growth to maturity, the material has afforded every variation in size and

structure.
SPECIFIC CHARACTERS

Mature Form (plate 6, figs. 13, 13a, 20, 202). Atrypa reticularis
is so widely distributed, historically and geographically, in paleozoic
faunas, and is so familiar to paleontologists, that a detailed description
here of its mature form is unnecessary. It is sufficient to remark that the
prevailing expression at this locality does not precisely conform to the type
of A. reticularzs, but is more nearly that variety described by Professor
Hatt (Paleontology of New York, Vol. 2, p. 271, 1852) under the name
Atrypa rugosa. This is evident from the development of the varical lamel-
la, which, over the plications, are infolded into nearly tubular processes,
sometimes produced at a strong angle from the shell to a length of a milli-
meter or more. On the varices, the plications are covered by fine concentric
wrinkles. The average size of mature individuals, 25x25 mm., is less than
that usual to the species, when occurring in later, especially Devonian,
faunas.

Incipient Form (plate 6, figs. 12, 12a, 15, 15@). The initial shell,
or the actual inchoate period in its formation, is not known to us. The in-
cipient shell of our series is very small, and can be but a few removes
from the initial stage. As just observed, it measures 2.25 mm. in length by
3 mm. in width, and shows but two concentric striae, or growth-varices, with
a correspondingly slight development of the deltidial plates, so that we are
inclined to regard this shell as but two stages advanced from the actual

SILURIAN BRACHIOPODA 53

inception of the shell. The test is flat, both valves being shallow and
depressed toward the anterior margin; the ventral beak high and erect,
the dorsal beak inconspicuous and rounded. The foramen, which is
undoubtedly triangular in the initial shell, has, at this stage, its basal angles
slightly rounded by the faintly developed deltidial plates. The plications
are six in number on the ventral, and five on the dorsal valve, the middle
one of the latter not reaching so far toward the beak as those adjoining it,
and toward the anterior margin being depressed below the lateral portions
of the shell. General outline subcircular or subpentagonal, as in the full-

grown shell.
DEVELOPMENTAL VARIATIONS

General Form and Outline. Embryos of less than 3 mm. in length
are more nearly circular in outline than at any subsequent period of the
existence of the individual. Directly thereafter, the hinge-line represents
the greatest diameter of the shell, and the outline becomes subpentagonal, a
feature which is more apparent in young individuals having between 3
and 10 mm. length, as the increasing rotundity of ‘the shell with the
approach of maturity has a tendency to obscure, in a measure, this out-
line. At the earliest stage studied, the dorsal valve is distinctly depressed
along the median line, forming a sinus containing a single plication which
does not reach to the beak (plate 6, fig. 142). This,sinus gradually
becomes shallower, and the plications are increased by intercalation until
they are three in number (fig. 144). In the next stage, all evidence of a
sinus upon the anterior margin disappears, leaving it even and straight
as shown in figure 14c ; then the anterior edge becomes reflexed, showing, in
subsequent stages of growth, a fold where there had previously been a
sinus, this fold bearing at first three, then five, and eventually, in the
mature individual, seven plications (fig. 14d, ¢, f). This very remarkable
reversion of the fold and sinus relatively to the valves which bear them, is
also seen in the species Rhynchotreta cuneata and, in all adult specimens,
may be clearly traced upon the earlier or embryonal portions of the valves.

* .

54 MEMOIRS OF THE STATE MUSEUM

Beak. In the first stage, the ventral beak is high and slightly re-
supinate, exposing the foramen in an inclined plane. It gradually shortens
and becomes erect, and when the shell attains a length of 8 mm,, it is bent
forward, the cardinal area being slightly incurved. Thereafter, the inflec-
tion of the area increases, concealing first the deltidial plates, and finally —
the foramen, until, in maturity, the beak lies appressed upon the embryonal
sinus of the dorsal valve.

Foramen. In the initial shell, this is undoubtedly triangular and
free from deltidial plates. With the starting point of our series, however,
plates have begun to develop, thus narrowing the pedicle-aperture, and
rounding its basal angles. With the growth of the plates more rapidly
along the lower portion of their inner edges, the foramen shortens quickly,
while narrowing but slowly, assuming in the second stage (fig. 16), a
lanceolate, in the third stage (fig. 17), an oval, and in the fourth stage (fig.
18), a broadly circular outline. In the last two of these stages, the deltidial
plates have come in contact with’ each other above the apex of the dorsal
valve, and the pedicle-aperture itself has, from the second, if not from the
first stage in the series, encroached upon the apex of the valve, so that, as
it attains a circular outline, one-half its periphery is formed by the substance
of the valve itself, and the other half by the deltidial plates. From this
stage upward, there is no apparent change in the actual dimensions of the
foramen, and, therefore, with the growth of the shell it becomes relatively
much smaller. It appears, however, that with the incurving of the cardinal
area and the concealment of the deltidial plates, the foramen becomes more
and more inclosed by the apical portion of the valve, and it may be that
actual contact with the deltidial plates in the last stage of development is
lost. In this final stadium, with the procumbent position of the ventral
beak upon the dorsal valve, the plane of the foramen is parallel to the sur-
face of the dorsal valve, and the aperture is therefore lost to sight, or visi-
ble only at its upper edge.

SILURIAN BRACHIOPODA 55

Plications. Of the five and six plications visible upon the youngest
member of the series, three or four appear to exist on that portion of the
shell included within the earliest growth-line, i. e., presumptively, the initial
shell, and they increase by intercalation until, in the adult, the average num-
ber is about sixty for each valve. Concentric lines of growth follow each
other with unusual rapidity, particularly in early life

Summary. Atrypa reticularis, in the development of its beak, fora-
men, and deltidial plates, is in essential harmony with the other uniforami-
nate shells here discussed. The reversal of the fold and sinus is an interest-
ing but not unique feature, and by the time it has been completely effected
many of the characters of maturity have been assumed. From the degree
of exposure of the foramen, we judge that the animal remained attached
by its pedicle up to adult growth, but with full maturity and the approach
of senility, the pedicle must have become atrophied and the animal set free.

Retzia EvAx, Hall, 1863
Pate V, Fics. 1-9

— —-, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 160. 1879.
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 302. 1882.

In this species, the superficial features have been found of much more
permanent character than is usual in the plicate brachiopoda from this
horizon. Not far from three thousand individuals have been-examined, and
these show a variation in size from a length of 1 mm. and a width of .8 mm.,
to a length and width of 25 mm. Throughout the younger stages in this
series of variations, the feature of primary importance in distinguishing
the embryo of this from those of other species, notably Retzza sobrina
Rhynchonella indianensis, and Rhynchonella whitz2, is the sinus which exists
on both ventral. and dorsal valves; and of much accessory value, the com-
paratively slight variation in the number of the plications on the lateral
portions of the valves. These features will be presently adverted to more

at length.

56 MEMOIRS OF THE STATE MUSEUM

SPECIFIC CHARACTERS

Mature Form (plate 5, figs. 2, 2a). Shell ovate, generally longer
than wide, both valves almost evenly convex, and of about the same depth.
Anterior margin generally slightly emarginate, on account of the median
sinus which exists on both valves. In rare instances, a low. median fold is
developed near the margin of the dorsal valve.

Ventral valve with the beak much elevated above the dorsal, and in-
curved, so that the plane of the foramen is parallel to the axial plane of the
shell. Foramen circular, or slightly subtriangular; deltidium generally
obscure on account of the infolding of the beak, but showing the triangu-
lar deltidial plates.

Dorsal valve regularly arcuate, except at the posterior extremity, where
the beak is closely incurved beneath the ventral umbo. The median sinus
usually carries from three to five plications, but sometimes becomes filled up,
in advanced growth, by the crowding of these plications. Ventral valve
with a well-marked sinus, generally bearing three plications. The sinal pli-
cations on both valves take their origin in front of the beak, and are of
interstitial growth, a fact which does not hold true for any of the other
plications. The surface is marked by from eight to twelve rounded, con-
tinuous plications on each side the sinus of either valve, all of these ex-
tending to the beak, with the possible exception of the more obscure ones
on the cardinal slopes. Only in rare instances and abnormally, do these
plications increase by interstitial addition. Imbricating lines of growth are
often present, and fine concentric stria are sometimes discernible.

The mature individuals of Retzéa evax divide themselves into three
groups, based on their relative proportions :

(a) Normal form, in which the length and width are equal.

(4) Long form, in which the length is greater than the width.

(c) Broad form, in which the length is less than the width.

In frequency of occurrence, the form (6) almost equals the normal,

SILURIAN BRACHIOPODA 57

while the form (¢) is more rarely met with. The form (4) is also of remark-
able persistence, and starts so early in the life of the individual as to sug-
gest a distinct genetic impulse.

Variations from the normal development. These are to an
unusual degree very slight, and may be classed as follows :

Obesity, which apparently occurs only when normal full growth has
been attained.

_A tendency to asymmetry in development of the sinal plications. A
marked illustration of this is afforded by an individual which, in repairing .
an injury to its shell, has abruptly developed six plications on one side of
the sinus, in continuation of three and to correspond with three on the
opposite side.

The absence of plications in the sinus. This is a feature of
rare occurrence, and is undoubtedly an infantile character retained
in later stages of growth. A single individual of immature growth
affords an illustration of a peculiar abnormality, indicating a reversal
in the growth to an embryonic condition. This shell (plate 5, figs.
3, 3@) has grown to a certain size and normally developed its plications,
but an abrupt period has been placed to their development, and over the
entire anterior portion of the individual, in front of a stout varix, the sur-
face of the shell is almost smooth. This is the exact counterpart of that
mode of growth, to which attention is called under other species, where
-the smooth embryonic condition of the shell seems to be prolonged for
more than the usual period of immaturity, and the mature features are
thereupon abruptly developed after the formation of a sharp growth-line.

DEVELOPMENTAL VARIATIONS
Our series of individuals illustrating the embryological changes in this
species is so complete as to show by almost imperceptible gradations the
entire chain of development from very near the starting point up to matur-

ity. This series begins with an individual measuring 1 mm. in length and
8

58 MEMOIRS OF THE STATE MUSEUM

.8 mm. in width, and at this stage of growth, the incipient shell has mani-
festly not received much increment. That this, however, is not the actual
primitive shell seems proven by indications of two very indistinct con-
centric growth-lines, and by the presence of faint radiating plications near
the anterior margin, between the second growth-line and the margin itself.
It is very probable that the incipient shell consisted of that portion of the
individual (plate 5, fig. 1) lying within the first growth-line, and as
this would make its size about .5x.4 mm., this fact in itself is sufficient
apology for our not having detected the earliest stage of its development,
even if other causes had permitted its preservation.

Beaks. In all normally developed individuals less than 5 mm. in
length, the beak of the ventral valve is erect and exsert. At about this
stage of growth, a tendency to apical incurvature is manifested, which
increases up to maturity, when, under normal development, the entire
umbo is evenly incurved, concealing the deltidial plates and often much of
the foramen. On the dorsal valve, the beak is quite obscure in the young-
est forms, and in later stages of growth is concealed beneath the deltidium
or incurved beak of the opposite valve.

Foramen. This appears first as a simple triangular opening, its
apex reaching to, but nof truncating the apex of the umbo, and it is re-
tained in this condition until the shell attains a length of at least 3 mm.
of normal growth. At this age, the deltidial plates begin to form, making
their first appearance as two minute triangular lamine, taking their origin
in the basal angles of the foraminal triangle; and giving the foramen a
lanceolate outline.

By increments to their internal edges, these plates presently come in
contact with each other, truncating the interior basal angle of each, the
plates being, from this period onward, in progressive symphysis. The incre-
ments to these plates are made more rapidly at and about their interior
angles, and asa result, the foramen assumes successively an elliptical, an

SILURIAN BRACHIOPODA 59

oval, and a circular outline. _The circular curve of its upper extremity
is caused by a slight encroachment upon the beak, and this in mature
age is so considerable that the primary or incipient shell is undoubtedly
wholly absorbed. The plane of the foramen remains, except in rare
instances, always vertical, although the deltidial plates become slightly
bent by the incurving of the beak. A striking exception to this rule is
represented on plate 5, figure 9, where an individual which has reached
early maturity shows the senile feature of a beak incurved to such a degree
as to almost obscure the foramen. On approaching maturity, the deltid-
ial plates appear to become anchylosed along their exterior edges, with
the shell itself, the line of union being marked with a low ridge, and we
have never seen them displaced by any distortion of the shell, as so often
occurs in Calospira disparilis.

Sinus. As already noticed, the sinus, in the elementary shell, begins
as a low, smooth depression, equally strong on both valves, and extending
almost to the beak. It gradually becomes filled by the radiating plications,
which appear first at the sides, and increase toward the middle, never
becoming, normally, more than six. None of these plications reach the
apex of the shell.

Plications. On the latera of the shell, these seem to appear simul-
taneously, as shown in figure 1, where three on each side make their
appearance at the same stage of growth. This number is subsequently
increased to six or eight on each side in mature forms, sometimes becoming
greater in old age. _

Internal Apparatus. The brachial supports in this species consist
of spirals, coiled in the transverse axis of the shell, with their bases facing
each other. In the mature individual, the number of coils is from eight to
ten. The spirals are connected by an angular loop, the branches of which
take their origin on the dorsal limb of the basal coils, and are directed ven-
trally and backward to beyond the axis of the interior cavity, forming at

60 MEMOIRS OF THE STATE MUSEUM

their junction not a simple angle, but a miniature sadd/e, from the posterior
extremity of which extends straight backward a little spiniform process.
The number of coils in these spires varies with the age of the shell. In
preparing a series to show the development of these structures, it appears
that the shelly ribbons composing the spirals not only make fewer coils in
early life, but that these are of exceeding tenuity in the primary stages of
development. The accompanying figures show the extremes of develop-
ment noticed in these respects, figures 1 and 2 representing the character of
the supports in the mature condition, and figure 3, the spirals as developed
in an individual having a length of 2.5 mm., where the ribbon makes but

Fig. r. Fig. 2. Fig. 3.

Fics. 1, 2, 3. DEVELOPMENT OF INTERNAL APPARATUS IN RETZIA EVAX.

two revolutions. As far as can be ascertained, the /oof undergoes no
essential modification in these early stages, though its precise character
in the example from which this drawing has been made, was not determined,
but has been drawn in. The same arrangement, however, has been seen in
an individual of but slightly larger growth.

The growth of these spirals consists, primarily, in the addition to the
number of coils, and, secondarily, in the thickening of the ribbon. In the
first case, the increase in number must take place by addition to the apices
of the coils, and, therefore, the embryonic or primary coils of the ribbon
must be wholly concealed by later depositions upon them, both in length
and width. The apparent looseness of the coils in their primary condition

SILURIAN BRACHIOPODA 61

must, however, be regarded as largely due to magnification, the distance
between successive coils being actually not so great as the distance
between the apical turns of the ribbon in the mature spiral.

RETZIA SOBRINA, Sp. Nov.

PLATE V, Fics. 10-16
Rhynchonella whitit, Hall, zn part.

In the examination of alarge number of the specimens which have usu-
ally passed under the name of Rhynchonella whit, we have become con-
vinced, that, aside from the individuals which agree with the types and the
description of the species, there is a series of shells, which, in the mature
state, may be readily confounded with immature stages of R. whz¢z7, but in
their immature condition are readily separable from this species, and form, of
themselves, a satisfactory and well-defined developmental series. The simi-
larity of these examples with RX. whz¢cz is found in the general outline, the
strong, simple plications approximately the same in number, and the usual
two plications on the median fold. The external differences, however,
‘in the new species, are these: the plications on the fold may be one or
three, and whatever their number, the fold is always depressed, in most
instances even to obsolescence, and the plications upon it are low and often
faint. The foramen, also, is circular in maturity, with perfectly developed
deltidial plates, and the surface of the valves usually conspicuously marked
by fine, crowded, concentric growth-lines. Internally, the difference is more
emphatic, as carefully prepared specimens show well-defined spirals having
their apices near the lateral margins, as shown on plate 5, figure 12.
While disavowing the intention of describing new species as remote from
the purposes of this paper, we have, for convenience’s sake in utilizing this
form for our work, to which it makes no unimportant contribution, desig-
nated it as above, as no doubt exists of its specific value.

Retzia sobrina, one of the more abundant of the Waldron brachiopoda,
is itself subject to some variation, more considerable indeed, than that no-

62 MEMOIRS OF THE STATE MUSEUM

ticed in either of the species Rhynchonella whittt and R. neglecta. Our
material represents all developmental stages between the limits of these
dimensions; 2x1.6 mm. (incipient shell) and 7x6.5 mm. (maturity), In
its youngest stages, it shows a certain degree of similarity with Retzza
evax, especially in the sinus on both valves, and in the sinal plications,
The greater number of the latter in 2. evax, as well as the more numerous
lateral plications, will serve to obviate confusion here.

SPECIFIC CHARACTERS

Mature Form (plate 5, figs. 11, 11a, 116). Shell small, rotund, in
outline broadly ovate to subpentagonal. Valves of equal convexity.

Ventral valve with umbo prominent, attenuate, erect, and slightly in-
curved at the apex; cardinal margins not excavate, sloping with a faint
curve to the sides, whence they round to the anterior edge, which, in the
sinal region, is nearly straight; cardinal area distinct ; foramen circular ;
deltidial plates prominent.

Dorsal valve subcircular in outline, arched in the umbonal region ;
beak well-defined, apex concealed. In the umbonal region, the median
portions of both valves are slightly more convex than elsewhere, but this
prominence disappears toward the margins, the valves becoming slightly
flattened and depressed on the median region near the anterior margin,
making a low sinus on the ventral, and a low, depressed fold on the
dorsal valve. Both fold and sinus may bear one, two, or three small, often
faint and unsymmetrically developed plications, the strongest of which
may have its origin in the umbonal region, while the others rarely extend
more than half-way across the shell. On each of the latera are four or
five strong, angular, simple plications, making thus from nine to thirteen
plications on each valve. The increase in these takes place altogether on
the fold and sinus, the full quota of lateral plications appearing early in the
history of the individual. The plications are covered by numerous fine,
concentric growth-lines more noticeably developed near the margin, and at
intervals becoming varicose.

SILURIAN BRACHIOPODA 63

The spiral brachial supports each make about five revolutions, which
come to an apex near the lateral margins.

Variations from the normal mature form. An elongate form with an
unusually high and straight beak is not of rare occurrence, and is the result-
ant of a very completely represented series of embryonic stages. The spe-
cies is also subject to odese growth, resulting from two sources ; (a) general
internal thickening of the shell, (4) marginal thickening. Both are the
result of post-adolescent or senile growth, the former producing a round,
full, plethoric shell, the latter giving the shell a truncate appearance.

Incipient Form (plate 5, figs. 10, 102). The first stage of growth
represented in our series measures 2 mm. in length by 1.6 mm. in width.
The valves are subequally convex, somewhat depressed anteriorly. Outline
broadly ovate.

Ventral valve with beak high, erect, and subacute; cardinal slopes
broad, steep, and slightly excavate ; pedicle-aperture subtriangular, rounded
at the apical angle, and also slightly, at the base, by the already developed
deltidial plates. .

Dorsal valve subcircular, beak full, rounded, sides slightly appressed,
apex concealed. Surface of the dorsal valve marked by two thread-like pli-
cations which take their origin medially, just below the umbonal region;
thenceforward they rapidly diverge, forming the embryonal sinus, which is,
however, soon filled by two small plications. The latera each bear one
plication, of earlier age than the sinal, and later than the primary plica-
tions. On the ventral valve, the plications number the same, but the
embryonal fold or dorsum is more strongly marked than the dorsal sinus.
On both valves, indications of the mature fold and sinus begin with the
appearance of the sinal plications.

DEVELOPMENTAL VARIATIONS
General Form and Outline. There is'a gradual increase in convexity
and diameter with each successive growth-stage, until maturity is reached.

64 MEMOIRS OF THE STATE MUSEUM

The elongate variation from the normal seems to have retained through all
stages of growth the proportions of the normal embryo.

Beak and Foramen The erect, straight, subacute beak of the incip-
ient shell, in later growth becomes rounded and slightly arched or incurved.
The cardinal area in all stages of development is, however, high, exceptions
being made for the more extreme cases of obesity, where the deltidial
plates may be concealed, but the foramen is always exposed, and the beak
is never procumbent on the opposite umbo. The plates arising from the
base of the thickened foraminal margins, meet in such a manner as to leave
the foramen sharply acute below, and subtriangular. By their subsequent
upward growth and more complete union, the foramen becomes circular, the
lines of symphysis with the valves still remaining thickened.

In occasional instances, a probable slight displacement of the plates
outwardly along the median suture, gives them the appearance of sloping
away from the median to the lateral sutures.

Ca@LOsPIRA DISPARILIS, Hall, 1852
Pate V, Fics. 17-23

—— — , Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 162, pl. 25, figs. 39-43.
— eee Eleventh Ann. Rept. State Geol. Indiana, p. 303, pl. 25, figs. 39-43. 1882.

Although this species is one of the less abundant members of the
Waldron fauna, we have found several hundred immature individuals, the
youngest of which has dimensions of 2.5 mm. length by 2 mm. width, the
greatest size at maturity being 6.5 mm. length by 6 mm. width. The
species being, in its mature size, quite small, and, in its surface features, quite
simple, it does not afford such scope for variations through the later embry-
onic stages as many of its associated species and hence, it will be noticed,
that in surface sculpture, a permanency of character is retained through all
stages of growth. 7

SILURIAN BRACHIOPODA 65

SPECIFIC CHARACTERS

Mature Form (plate 5, figs.19, 19@,194). . Shell small, suboval or
cordate, often subpentagonal ; plano-convex, greatest’width along the hinge.

Ventral valve convex, depressed at the sides ;,beak exsert, and in old
forms arcuate.

Dorsal valve flat and sometimes depressed near the beak. The
ventral valve is marked by two very prominent plications which pass
along the deep median sinus, and are accompanied by two less distinct pli-
cations on each of the latera, making in all six plications, of which the
two nearest the hinge-line are sometimes obsolescent. The dorsal valve
bears a prominent median fold, and two well-marked plications on the lat-
eral portions of the shell. Toward the margin are concentric lamine of
growth. An average adult individual measures 5.5 mm. in length by 5 mm.
in width.

Variations in Outline. The individuals divide themselves into
two groups according to their outline :

(a) normal form.
(4) long form.

The first of these groups includes the great majority of all individuals
found, which are characterized by a relatively broad figure and subcircular out-
line. Members of the second group are comparatively few in number, and -
are elongate or subovate individuals. The long form (é) is well defined in
immature growth-stages, and appears to be a permanent varietal difference.

Abnormalities. A variation in adult shells, noticed only in rare
instances, is a tendency to an asymmetrical development in the plications, as
shown on plate 5, figure 18, where, by unequal growth upon the lateral por-
tions of the shell, the median plication on the dorsal vaive is deflected to one
side, and the corresponding median sinus on the ventral valve displaced,
the axial line of the shell being occupied by one of the strong plications

bounding the sinus. Another form of this asymmetry is manifested in the
;

66 MEMOIRS OF THE STATE MUSEUM

intercalary addition of a single plication on one side of the median plica-
tion of the dorsal valve.

A tendency to obesity is often manifested by the shell, at or before
reaching the average dimensions of maturity, when it may be supposed
that the full growth of the individual has been attained. This obesity is
produced by a rapid thickening of the shell at the margins, making the
anterior face truncate and forcing the ventral beak over the dorsal in the
same manner as if the valves were forced to open along the hinge. It is,
therefore, only in individuals which have reached this obese condition that
the ventral beak is incurved.

DEVELOPMENTAL CHANGES

The character of the primal or elemental shell may be seen from a
single example (plate 5, fig. 18), in which the plications are abruptly
developed at a distance of 1.5 mm. from the apex, and, presumably, that
portion of the shell within this limit, represents approximately the size of
the original embryonic shell. This portion of the individual is quite smooth,
and shows but a trace of the median fold and sinus. As already observed,
there is a marked permanency in the surface features of the species from
early youth to maturity. The smallest individual obtainable bears, as in
the mature condition, six plications on the ventral and five on the dorsal
valve, though those near the hinge-line are quite faint.

The éeak is prominent and exsert, except in obese shells, where it is
incurved. In the earliest stage where the character of the foramen is well
preserved the individual has a length of 4 mm. and a width of 3.5 mm.
Here it is seen to be elongate-oval, the deltidial plates having formed to
such a degree as to be in contact with each other and to have anchylosed,
so that the median suture is detected with difficulty. The lateral sutures |
always remain distinct even to maturity, and it is evident that the union of
the plates with the shell along these joints has not been as firm as in many
species, as it is not infrequently found that the plates have been displaced

‘ SILURIAN BRACHIOPODA . 67

and lost. Careful search among the smallest individuals has shown no trace
of the inceptive triangular outline of the pedicle-groove existing in other
species before the formation of the deltidial plates. It is an important fact
that the foramen begins to assume its mature condition so early in the
history of the shell, although its development was evidently in conformity
with the general type.

The subsequent development of the deltidial plates changes the form
of the foramen to that of a circle, as shown in figures 22 and 23. Inthe
early life of the shell, the plane of the foramen is in, or parallel to, the
axial plane; at maturity, before any obesity or senile thickening takes
place, the foramen, in becoming less elongate, truncates the apex of the
valve, and makes a large angle (sometimes almost 90°) with the axial
plane ; subsequently, with increase in obesity, it becomes again more nearly
parallel to this plane. In the last condition, the deltidial plates are curved
inwards, and often to a large degree concealed.

MERISTELLA RECTIROSTRA, Hall, 1882
PLATE VII, Fics. 4, 5, 11-13
— —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 301, pl. 27, figs. 10-14. 1882.

This small species is one of the less abundant of the brachiopods of
this fauna, and probably has often been confounded with undeveloped indi-
viduals of Merdstina nitida. It presents, however, adult features which will
not allow it to be confounded with that species, and although some. diffi-
culty arises in separating the diminutive forms of the two species, IZ. rect?-
rostra is characterized by the absence of deltidial plates in every stage of its
existence. ?

The series representing this species does not include stages of growth
as early as in some of the others, but is sufficiently complete to permit the
statement that, were younger forms accessible, they would probably add
little to our knowledge of the developmental changes. The series begins

68 MEMOIRS OF THE STATE MUSEUM

with individuals having a length of 2.5 mm. and a width of 1.75 mm., the
adult form measuring 9 mm. in length by 7.5 in width.

In all stages of growth earlier than that approximately indicated by a
size of 6x4 mm., it is very difficult, and from our observations impossible,
to draw the line of separation between this species and JZ. xztéda, and the
fact which has been demonstrated for Orthzs elegantula and O. hybrida, viz.;
that in the earliest growth-stages no specific differences are .manifest, will
probably be found to hold good for these two species also.* And in the
latter case a considerably larger size is attained by the embryo forms than
is reached by the former species, before the differential characters are
assumed, This is due to the fact, that these two species, when mature,
have essentially no surface sculpture, and differ less in general form and
outline than do the mature individuals of Orthes.

SPECIFIC CHARACTERS

Mature Form (plate 7, figs. 12, 12a, 126). Shell subpentagonal or
ovate ; beak erect, acute, and prominent, rapidly widening toward the base.
Lateral margins nearly straight for about one-third the length of the shell,
thence rounding to the anterior margin. Valves about equally convex,
giving the shell a sublenticular contour.

Ventral valve with attenuate, straight, or slightly arcuate beak. Fora-
men triangular and without deltidial plates.

Dorsal valve more nearly subpentagonal in outline ; beak incurved into
the foramen of the ventral valve.

Surface smooth, or in rare instances. showing a faint pseudo-punctate
appearance which is entirely superficial. Dimensions of average adult
97.5 mm.

*In accordance with the intention already expressed, not to enter into discussions of
synomymy and generic values, we have followed Mr. Hall in referring these species (7. rectz-
rostra and M. nitida) to different genera. We believe, however, that the character of the loop
will prove them to be congeneric.

SILURIAN BRACHIOPODA | 69

Incipient Form (plate 7, figs. 4, 4). Shell measuring 2.5x1.75
mm. Oval, proportionally longer and narrower than in the adult state.
Beak elevated, acute, straight. Foramen of the ventral valve very broad,
triangular, extending to the apex. Dorsal beak full, rounded and incon-
spicuous. Shell convex just below the beak, becoming depressed toward
the anterior margin.

DEVELOPMENTAL VARIATIONS

General Form and Outline. In the incipient stadia of growth, the
shell is extremely elongate and quite perfectly oval ; the beak of the ventral
valve is relatively broad, its lateral margins having a slight outward curve. |
With growth, the shell broadens, and the ventral beak becomes more atten-
uate while the greatest width of the shell, instead of being at or below the
middle, comes nearer the hinge-line.

Beak. From being erect, straight, and relatively broad in the ventral
valve, at the outset, it becomes, at maturity, narrow, attenuate, and slightly
incurved toward the apex.

Foramen. In the earliest observed stage, the foramen is a broad,
triangular opening, covering nearly the entire cardinal area, reaching, but
not encroaching upon the apex of the valve. In subsequent stages of
development, this opening narrows with the narrowing of the beak, but, as
at no stage deltidial plates are developed, the contraction is due to the en-
croachment of the cardinal portions of the valve along the foraminal mar-
gins. The interesting fact of the persistent absence of deltidial plates
throughout the entire existence of the individual, may be interpreted as a
retention to maturity, of a character embryonic in allied species; the small
size of the mature shell and the very slight incurvature of the ventral
beak also contribute to the embryonic expression of the species.

70 MEMOIRS OF THE STATE MUSEUM

Meristina NitTrpA, Hall, 1843
PLATE VII, Fics. 6-10

—— —-, Hall. Twenty-eighth Ann. Rept. State Mus. Nat. Hist., p. 160, pl. 25, figs. 1-7. 1879.
—— —, Hall. Eleventh Ann, Rept. State Geol. Indiana, p. 300, pl. 25, figs. 1-7. 1882.

Meristina nitida is a very abundant and characteristic fossil in the
Niagara fauna of central Indiana, reaching a much greater development
both in size and numbers than in the New York outcrops of the formation.
The individuals vary in size from a length of 2.5 mm. and a width of 1.75
mm., to a length of 25 mm. and a width of 22 mm., and the series represent-
ing these variations is, on account of the abundance of specimens, very
complete within these limits. It is, however, a noticeable feature of the
species that in most respects, except size, the characters of maturity are
assumed early in the life of the individual, and as the form is essentially
devoid of surface sculpture, the interest in its development rests to a larger
degree than usual upon abnormalities in individuals either mature, or ap-
proaching maturity.

SPECIFIC CHARACTERS

Mature Form (plate 7, figs. 9, 9a, 90). Shell broadly subpentagonal
to ovoid; beaks extended and more or less prominent.

Ventral valve with the greatest convexity at about one-third the length
of the shell in front of the beak. Beak arched, incurved over the dorsal
valve ; apex evenly truncated, the circular foramen lying in a vertical plane.
Cardinal slopes extending for more than one-half the length of the shell.
A low median depression is noticeable on the younger portions of the valve.

Dorsal valve with a similar convexity ; beak incurved and concealed.
A very low and inconspicuous median elevation corresponds with the
depression on the opposite valve.

Surface smooth or with fine concentric strize and a few conspicuous lines
of growth toward the anterior margin. Average individuals measure 20x
16 mm., large examples not infrequently 25x21 mm,

‘

SILURIAN BRACHIOPODA 71

Variations in Outline. Two very distinct groups of forms are evi-
dent in this species, in one of which, (@) xormad/, the outline of the mature
shell is obcordate or subpentagonal. When immature, the anterior margin
is evenly circular, but in all cases the proportion of length to width is essen-
tially the same. Probably five-eighths of the specimens found belong to
this group.

In a second group, (4) dong form, the shell is elongate-spatulate, and
proportionately deeper than the normal, but, with a single exception, indi-
viduals have not been observed to exceed a length of 10 mm. and a width
of 7mm. This variation is so persistent that it appears to be well founded
genetically, and not merely an occasional occurrence. Tracing backward
from the mature shell to the earlier stages of development, we find both
this and the normal form merging into each other, hence both have had a
similar starting point. The long form, however, reaches maturity of
development at a very early age, and never approaches the size or propor-
tions of the normal adult. A tendency to odeszty is especially noticeable in
the group (4), the majority of such individuals being below the normal full
growth.

A single adult example shows traces of broad, rounded plications on
each side of the fold and sinus, a singular condition in a species uni-
formly non-plicate.

Incipient Form (plate 7, figs. 4, 4a, 10, 102). Shell 2.5 mm. in
length by 1.75 mm. in width ; elongate, oval ; beak elevated, straight, acute.
Pedicle-aperture of the ventral valve very broad, triangular, extending to,
but ‘not encroaching upon the apex. Dorsal beak full, rounded, but incon-
spicuous. Valves convex just below the beaks, becoming depressed toward
the anterior margin. The shell is proportionally much narrower than the
adult form.

The starting point of our series is precisely the same form of shell as
that taken for the incipient stage in the species Merdstella rectirostra.

72 MEMOIRS OF THE STATE MUSEUM

Under the discussion of that species, reference has been made to the impos-
sibility of separating these two species, in their earlier stages, and we are
left to regard the impression of the specific characters as .of subsequent

development.
DEVELOPMENTAL VARIATIONS

The surface characters being unvariable, the important changes in
development are confined, as far as observable, to the pedicle-aperture and
deltidial plates. As already observed, the beak is incurved so early in the
history of the individual that these embryological changes can be observed
only in very young specimens. This incurvature of the ventral beak
appears to become fixed earlier in the normal than in the elongate form,
For example, figure 10, plate 7, represents an elongate individual with a
length of 2.5 mm. and a width of 1.5 mm., with an open triangular foramen,
and no apparent development of the deltidial plates, but the normal form of
the same size has the plates developed, the foramen nearly circular and the
beak incurved. In the condition represented in this figure, the embryos of
this species are readily confounded with Jerdstella rectzrostra, in which the
triangular aperture is retained until maturity. The latter species is, how-
ever, distinguishable in all the later stages of its existence by the body of
the shell being broader and the ventral beak narrower and more attenuate.

Individuals which show the deltidium in its different phases are diff-
cult to obtain on account of the tendency of the beak to incurvature as
soon as the plates begin to form. An individual is represented in figure 7,
plate 7, of somewhat abnormal height of beak, showing an intermediate
stage of growth in the plates and the formation of the foramen; and in
figure 8, an individual of the same size with the foramen circular, and the
deltidium completed and concealed by the infolding of the beak.

SILURIAN BRACHIOPODA 73

WHITFIELDIA MARIA, Hall, 1863
Prate VII, Fics. 1-3

Mertstella Maria, Hall. Trans. Albany Inst. vol. iv, p. 212. 1863.
Meristina Maria, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 159, pl. 25,

figs. 8-12. 1879.
—— —,, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 299, pl. 25, figs. 8-12. 1882.

* Whitfieldia tumida (Dalman Sp.), Davidson. Supp. Brit. Sil. Brach., p. fo7. 1882.

This species may be regarded as presenting a general external form and
effect diametrically opposed to that in Werzstella rectirostra. To the erect,
attenuate, acute beak, open pedicle-aperture, shallow valves, and asinuate
anterior margin of the latter, the full, rounded, incurved beak, concealed
cardinal area, ventricose valves, and strongly sinuate anterior margin of
W. marta are strongly contrasted. Between the mature characters of these
two species, Weristzna nitida is conspicuously mediate.

Immature specimens of W. marza are far from abundant. Indeed,
our series shows only about thirteen different grades of development, and
the smallest individual which can be referred with certainty to the species
measures 6x6 mm. (adult 29 mm. in length by 32 mm. in width),

We have, however, assigned to the species a minute embryo measuring
-75x.75 mm., and if this is correctly done, the embryos of this species in the
earliest stages of growth differ from those of the other non-plicate species
here discussed in a much stronger tendency toward a circular outline.

The beak of the ventral valve becomes incurved, and the cardinal area
obscured very early, so that the discussion of the development of these
parts is necessarily much curtailed.

* The late Mr. Davidson identified the Waldron species with the Atryfa ¢umida of Dalman, —
the type of his genus Whitfieldia. We use the term WAztfieldéa here for convenience, but if the
identity of the American and European forms is established, 4¢ryfa tumzda will have been taken
as the type of two (Meréstina, Hall, Whitfieldia, Davidson), if not of three (dJeréstel/a, Hall)
genera.

10

74 MEMOIRS OF THE STATE MUSEUM

SPECIFIC CHARACTERS

Mature Form (plate 7, figs. 3, 3a). Shell comparatively large, ven-
tricose, broadly ovate or subpentagonal. ;

Ventral valve gibbous in the umbonal region, with a low, broad dorsum
extending from the umbo to near the middle of the valve, where it becomes
flattened, sinuate, and at the anterior margin is reflected dorsally into a
linguiform extension. Beak closely incurved over the dorsal valve, fully
concealing the foramen. Cardinal slopes angulate, and slightly excavate.

Dorsal valve evenly convex, somewhat gibbous, strongly arcuate trans-
versely along the dorsum, which becomes elevated into a low fold, deeply
emarginate in front for the reception of the extension from the opposite
valve. Beak obtuse, incurved and concealed.

Surface smooth, marked by concentric growth-lines near the margin.

Occasionally, individuals of large growth show a greater length than
breadth, presenting an elongate form, but this variation seems to be due to
more rapid axial growth after the attainment of adult size, and does not
manifest itself in the incompletely developed shells.

Incipient Form (plate 7, figs. 1, 1a, 2, 2a). The minute shell which
appears to have been the initial form for the species, has a circular outline
and depressed convex valves. The ventral valve is evenly convex, with
the beak erect, short, and broad. The cardinal area is low, the foraminal
aperture triangular, reaching to, but not encroaching upon the beak. The
deltidial plates are absent. Dorsal valve with the beak not incurved but
inconspicuous. Neither valve bears any trace of. a median elevation or

depression.
DEVELOPMENTAL VARIATIONS

General Form and Outline. An inclination toward a lenticular form
and circular outline is noticeable in all immature individuals. Until a size
of about 18x18 mm. is attained, there is rarely, if ever, any trace of the
strong marginal fold of maturity.

SILURIAN BRACHIOPODA | 75

Beak. The low but erect ventral beak of the initial shell, has, in the
next stage of growth, become inflected and obtuse, not, however, so as to
conceal the foramen, which remains apparent above the apex of the dorsal
valve, until the rapid increase in convexity, which immediately precedes
maturity, sets in. Thereafter, the ventral beak becomes more closely in-
curved, and thrust over upon the dorsal valve, to the loss of all external
trace of the cardinal area.

Foramen. The elemental hiatus is shown in the initial shell and the
subsequently developed deltidial plates in the next growth-stage. In the
latter case, the foramen has become nearly if not quite enclosed and has
also encroached upon the apical portion of the valve, which forms about
one-half its periphery. In all subsequent stages of growth, the deltidial
plates are concealed, and whatever portion of the foramen appears there-
after above the dorsal valve is enclosed by the circumbonal tract. With
the approach of maturity, this gradually disappears, and at full growth,
every trace of it has become obliterated.

SPIRIFER Crispus, Hisinger, 1826
Puate VI, Fics. 6, 7

SPIRIFER CRISPUS, var. SIMPLEX, Hall, 1879
Pate VI, Fics. 4, 5

SPIRIFER BICOSTATUS, Vanuxem, 1842, var. PETILUS, Hall, 1879
Puiate VI, Fics. 1-3

Spirifera crispa, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 157, pl. 24,
figs. 6-12, 19. 1879. |

— ——, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 295, pl. 24, figs. 6-12, 19. 1882.

Spirifera crispa, var. simplex, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p.
157, pl. 24, figs. 1-5. 1879. )

— —- —,, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 286, pl. 24, figs. 1-5. 1882.

Spirifera bicostata? var. petila, Hall. Trans. Alb. Inst., vol. x, Abstract, p. 15. 1879.

— — —, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 297, pl. 27, figs. 8, 9. 1882.

The three forms which are here treated together, are closely allied in
all their general characters. It is in their initial stages, however, that the

76 MEMOIRS OF THE STATE MUSEUM

resemblance becomes more than superficial, for, in young shells of less than
2mm. in length, it is difficult, and sometimes impossiblé, to refer them to
any one of the three groups. A general expression of the common charac-
ters is furnished by the young of \S. crzspus, var. stmplex, illustrated by
figure 4, on plate 6.

Taking S. dzcostatus, var. petilus as the simplest form, the young shell
- is found to be nearly circular in outline, with a single, broad, median fold
_on the dorsal valve. Passing to .S. cr¢spus, var. simplex, of the same size,
the outline is seen to be broader, and there is an incipient plication on each
side of the median fold. The outline is still broader in S. crzspus, becoming
decidedly subelliptical, and the two lateral plications on the dorsal valve are
nearly equal in strength to the median fold. The surface ornamentation
consists of fine spinulose, or granulose, concentric striz, differing very little
in any of the three species.

In tracing the development of .S. dzcos¢atus, var. petzlus, the shell is found
to retain its embryonic characters up to full growth, neither materially
changing its form, nor adding to the primitive number of plications.
Likewise, .S. crzspus var. semplex changes very little except to increase in
width and add a pair of plications at maturity. Individuals of S. erzspus
develop parallel to the variety szmflex, up to a length of 5 mm.,, or until
about two-thirds the size of full-grown examples is attained. Subsequently,
more plications are added, increasing the number from three or five to
eleven, but otherwise, the general features of the shell are unchanged.
Even the relative convexity of the valves remains the same at all periods,

In the incipient forms, the cardinal line extends for about one-fourth
the width of the shell, and at maturity measures three-fourths of this width,
The foramen does not develop at the same rate; at first, it occupies one-
half or one-third of the ventral area, but advancing growth gradually
diminishes this ratio, until it is one-fourth or one-fifth the size of the hinge-
area. Two narrow, triangular, deltidial plates are present in full-grown

SILURIAN BRACHIOPODA 77

individuals, but they do not serve to close the fissure which remains
open in all stages of growth.

S. crispus, var. simplex reaches a width of 8 mm., and |S. crzspus often
measures 22 mm. in width. Occasionally, a specimen of S. crzspus of the
usual size is found with but seven plications on the dorsal valve, suggesting:
a very large example of the variety, or that the characters of the smaller
and simple form are sometimes continued far beyond the period when they
usually disappear. Also, the features both of the species and variety may
be combined in a single specimen, as one abnormal example has three pli-
cations on one side of the median fold and four on the other.

SPIRIFER RADIATUS, Sowerby, 1825
Piate VI, Fics. 9-11

Spirifera radiata, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 157, pl. 24,
figs. 20-30. 1879. -

— — Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 296, pl. 24, figs. 20-30. 1882.

The series of specimens representing the gradation in size from very
young to mature forms is quite complete, but, unfortunately, the characters
of the most interesting feature, the deltidium, are not well shown. The
foramen is usually but partially closed when the shell reaches nearly its full
dimensions, and at this period, the beak of the ventral valve is so incurved
and thickened that the detailed development of the deltidial plates is
obscured, and rendered difficult of interpretation.

This species has been so fully discussed in all its aspects, on account
of its wide geographical distribution and varied physical conditions, that a
diagnosis of the adult form is unnecessary in this place (vzde plate 6, figs.
Il, 11@).

Incipient Form (plate 6, figs. 10, 102). The smallest example yet
detected has a length of 1.5 mm. The specimen is not well preserved, and
the one used for illustration and description is somewhat larger, measuring

78 MEMOIRS OF THE STATE MUSEUM

2mm. in length. The differences appear to be so slight that the characters
of the larger may well be applied to the smaller individual.

The shell is nearly circular, flattened, and with the beaks not incurved,
but directed outward. The area of the ventral valve is broad, triangular,
open, and extends nearly the entire length of the cardinal line. The incip-
ient dorsal fold and ventral sinus extend nearly to the beaks, and on each
_ side there are about ten radiating stria. Radii are also present on the fold
and in the sinus.

DEVELOPMENTAL CHANGES

The changes in the shell from advancing growth are principally the
gradual widening of the valves, on account of the extension of the cardinal
line and extremities, and the incurving of the beaks, from the progressive
increase in the depth of the valves. From being circular in outline, the
shell slowly widens until it is one-seventh wider than long. The ventral
beak in old specimens is so arched over the area as to nearly conceal it, and
prevent the opening of the valves to any extent. In the early stages, the
depth of the conjoined valves is about half the length of the shell, while
in obese mature forms, the depth is equal to the length.

The deltidial plates first appear as narrow elevated laminz along the
sides of the fissure under the ventral beak. A specimen about half-grown
shows them as represented in plate 6, figure 9, consisting of triangular
plates approximately as in figure 3’, of the following diagram. They are
subsequently united along their inner margins, and rarely, in the material
at hand, can any appearance of a foramen be discovered. In old shells,
the growth and thickening of the deltidium makes it rugose, and it nearly
closes the area.

From an examination of a number of species of Sfzrzfer showing
considerable variety in the mode of development of the deltidium, we
believe that there is no esssential difference, and that all intermediate con-
ditions occur in this group, between the features represented in Spzrzfer7na

SILURIAN BRACHIOPODA 79

by Deslongchamps (see Summary), and the characteristic mode of develop-
ment in Zerebratula and Rhynchonella. The genus Sfircfer presents all
these stages. In some species, the area is apparently closed by growth
from the apex, and in others by the meeting of the deltidial plates at the
base of the area and inclosing a foramen as in Rhynchonella. Spirifer
niagarensts, S. perlamellosus, and S. cumberlandi@ are examples of the former
mode, and .S. s#/catus and approximately S. radzatus represent the latter.
Both conditions are reached by accretion along the inner edges of the del-
tidial plates. The initial state is represented by a narrow elongate lamina
on each side of the triangular area. Further growth produces a triangular
plate, and to the form of the triangle is due the apparent growth of the
deltidium from the apex of the fissure downwards, or from the base of the
fissure towards the beak of the ventral valve.

The accompanying diagrammatic outlines serve to illustrate the
changes and the final results.

Fic. 4. DetripiIAL DEVELOPMENT IN SPIRIFER.

Figure 1 represents an area in an early stage of growth, with a narrow
deltidial plate on each side, alike for each series.

Figure 2 shows scalene triangular plates, with the shortest side at the
base of the area. ‘

Figure 2’ shows plates with the two free edges more nearly equal.

80 MEMOIRS OF THE STATE MUSEUM

Figure 2” presents narrow triangular plates as in figure 2, but with
the shortest edges in the apex of the area.

In figures 3, 3, 3’, the growth has continued in the direction initiated
in the preceding stage, and the. apex of the area has been partially filled
from the internal thickening of the beak.

Figures 4, 4’, 4 show the completed deltidium with the circular perfo-
ration. The plates in figure 4 nearly close the area, while in figure 4” the
opening is nearly as high as wide. Further growth can now take place
only along the lower free edges of the plates.

Figure 5 represents the results of subsequent growth and thickening,
which have obliterated the evidences as to the mode of development,
and unified all three cases. The position of the foramen below the apex of
the area does not appear to be due to the approximation and union of

the deltidial plates, but to the lowering of the actual cavity of the beak.

from the natural thickening of the shell, so that the foramen, as in other
genera, is at the real termination of the ventral umbonal cavity.

It is seen that the manner of development is alike in each case, varying
only from differences in the form of the plates in the earlier stages. The
finished deltidium is also the same, although the methods of attaining the
result differ in each.

Figures 1, 2, 3, 4 are represented by »S. sadcatus,* and vary in no im-

portant particulars from the mode of development in 7Zerebratuda and.

Rhynchonella.

Figures 1, 2’, 3’, 4’ are partially represented by S. radzatus, although, in
this species, the circular foramen is usually obliterated by subsequent thick-
ening and growth, (Sze figures 10, 9, 11, plate 6.)

Figures 1, 2”, 3” are well shown in S. xzagarensis (figure 8, plate 6), and
the subsequent stages appear in mature forms of |S. ey teelloses: cand .S. cum-

* State of New York, Report of hie: State Geologist ox: the reat 1882, published 1883, wih ad
figs. 1, 2, 3.

SILURIAN BRACHIOPODA 81

berlandie. Other forms, notably those with elevated areas, such as .S. macro-
notus, S. medialis, together with Cyrtzna and Cyrtia, present considerable
differences in the completed deltidium, due principally, it is believed, to the
internal thickening of the beak and the growth of the transverse septum.

The deltidium of Sfzrzfer thus appears to be the exact homologue of
the same part in 7erebratula, Rhynchonella, etc., and to be radically differ-
ent from the so-called “ pseudo-deltidium” of Strophomena, Strophodonta,
Streptorhynchus, and allied genera, although this term has been currently
applied to both the Spiriferidae and the Strophomenide.

it

SUMMARY

OF

DEVELOPMENTAL CHANGES

Siz—E AND CONTOUR. Although the species described in the pre-
ceding pages present a wide variation in form and general appearance,
the nature of the changes which take place in the development of the
shell is remarkable in its uniformity.

In nearly every species, the inceptive state is represented by a
shell having a subcircular outline, with valves of slight convexity. This
phase usually disappears before the individual reaches a length of 1 mm.,
after which the specific characters are assumed, and are progressively
emphasized with each succeeding increment.

On comparing the incipient stage in these fossil shells with that
of recent brachiopods, as given by Mr. E. S. Morse for Zerebratulina,
and by Mr. W. K. Brooxs for G/o¢¢dédia, we find that, in respect to actual
size, there is a slight, though perhaps unessential difference. At the
earliest stage of growth figured by Morsz,* the shell has a length of
about .3 mm. and in the next stage represented, of approximately .6 mm.

*Memoirs of the Boston Society of Natural History, Vol. ii; On the Early Stages of
Terebratulina septentrionalis, pl. I, figs. 2, 3. 1869.

84 MEMOIRS OF THE STATE MUSEUM

The first two stages of the shell figured by Mr. Brooxs* represent
free animals, and measure .24 and .3 mm. in length, respectively. The
shell became attached by the pedicle only upon attaining a length of
2.5 mm.

Most of the fossil forms have furnished evidence, either from
actual elemental specimens or from the apical portions of subsequent
_incipient stages, that the true initial shell did not reach a size of more
than.5 mm. in length. Soon after this period, the characters of each species
become developed and impressed upon the shell more or less gradually.

Even such distinct groups as Ahyuchonella, Spirifer, Athyris,
Rhynchotreta, Anastrophia, Nucleospira, and the Meristoids, in their initial
stages, approach one another so closely that they can be determined
only from comparatively trivial features. They are alike in form, con-
tour, convexity, beaks, and cardinal area, and the only marked differences
are to be found in the faint indications of plications, striz, folds, and
sinuses. For species of some genera, as Orthzs, Whitfieldia, and Spirzfer,
even these characters are not determinative, and it is impossible to refer
certain embryos to their proper place.

From the foregoing statements, it would naturally be inferred, that
the species which, at maturity, present characters abnormal to the typical
structure, have been diverted from the harmony which existed in the
incipient stages, with the other members of the group. This has been
shown to be the case in all the reversed species above examined, belong:
ing to the genera Anastrophia, Strophonella, and Streptis.

Beginning with the initial shell having a circular outline and de-
pressed valves, we find that subsequent growth takes place about the
periphery, andein the majority of species, the convexity is gradually
increased until maturity is. reached. This assertion does not hold true

*Johns Hopkins University; Chesapeake Zodlogical Laboratory. The Development of
Lingula and the Systematic Position of the Brachiopoda, plates 1 and 2. 1879.

SILURIAN BRACHIOPODA 85

for such forms as the Strophomenide, which vary in convexity, either
very slowly or not at all, up to individuals about half-grown, when the
valves become more or less deflected and often concave. Such a re-
version in the shell is in conformity with the degeneracy which is traced
in the development of the cardinal area-and pedicle-sheath, mentioned
on a subsequent page.

The observations of Brooks and Morss, in the works just cited,
show, that in both the hingeless and the hinged brachiopods, as repre-
sented by Lingula and Terebratulina, the early stages of the shell
approach a subcircular outline, and Brooxs remarks (of. c7¢. p. 43), that
“the recent and fossil shells of the various species of Crania, Lingula,
Lingulella and Obolus, and other hingeless Brachiopods,’ furnish a series
of adult forms representing all the changes through which the outline
of Lingula pyramidata passes during its development.”

In these respects, then, uniformity is established in the embryology
of the ancient Silurian types and their modern descendants.

VALVES. The dorsal valve in young shells is smaller than the
opposite, and usually more depressed. These relations, as a rule, are
continued up to adult size, except that the ventral valve often increases
more rapidly in convexity, producing a consequent incurving of the beak
over the cardinal area, as in Orthis and Rhynchonella indianensts. Some
species present both beaks as incurved, a condition well represented in
Whitfeldia maria, Eichwaldia reticulata, Rhynchonella acinus, and R.
neglecta. \n Anastrophia, the comparative relations of the valves become
reversed from their initial condition, on account of the more rapid in-
crease in the depth of the dorsal valve, so that, at maturity, the dorsal
beak is much incurved, and often the umbo extends beyond that of the
other valve, although the beaks preserve their normal condition of
superposition.

Several of the species show an embryonal sinus in the dorsal valve,

86 MEMOIRS OF THE STATE MUSEUM

with a corresponding fold in the ventral, beginning soon after the initial
stage of the shell has been passed, and disappearing before the shell is
half-grown. Those forms presenting this feature to a marked degree
are Rhynchotreta cuneata, Rhynchonella whitit, R. indianensis, R. neglecta,
R. actnus, Atrypa reticularis, and Retzia sobrina. In Rhynchotreta cuneata
and Aétrypa reticularis (plate 4, figs. 12-15, and plate 6, figs. 12-14),
we have shown the gradual inception of this sinus, its maximum develop-
ment and obsolescence, and, finally, its reversion into a fold which there-
after persists and usually increases in prominence in all the succeeding
stages of growth. The embryonal sinus is not present in Sferdfer,
Anastrophia, Echwaldia, Whitfeldia, Meristina, Orthis, nor in the
Strophomenide. Such of these as show a dorsal fold or ventral sinus,
have them developed early in the growth of the shell, and they usually
increase regularly to the time when the full size of the shell is attained.

Breaks. The beak of the ventral valve in its earliest condition is
commonly erect, pointed outwards, and of a broad triangular form, while
that of the dorsal valve is small, not prominent, and lies in the longitudinal
axis of the shell. In all cases, the subsequent deepening of the valves
tends to incurve the beaks toward the cardinal area. The degree of in-
curvature varies greatly in the different species. MMerdstella rectirostra
shows a minimum, and Whztfieldia maria or Atrypa reticularis,-a maxi-
mum, and between these limits all intermediate conditions occur. The
usual degree of incurvature is presented in Spzrdfer radiatus, Retzia evax,
and the Rhynchonellide.

The outlines on plate 4, illustrating the profiles of the beaks in a
series of Rhynchotreta cuneata, represent an uncommon condition, for in
this species, the ventral beak, from its divergent initial position, gradually
approaches, and at maturity attains parallelism with the longitudinal axis of
the shell. It never becomes sufficiently incurved to conceal, to the slightest
degree, the deltidial area, while the initial dorsal beak becomes more

ES

SILURIAN BRACHIOPODA 87

and more incurved, until, finally, it lies entirely within the ventral umbonal
cavity.

Those species furnished with a circular apical perforation, as Atryfa
reticularis, Retzia evax, and Rhynchotreta cuneata, lose the initial point of
the ventral beak from absorption, due to the increase in the size of the per-
foration or to its final terminal position. In Atrypa reticularis, or Whit-
fieldia maria even both beak and perforation are destroyed, from the
forcing of the ventral beak into contact with the dorsal umbo, produced by
the great increase in the depth of the valves from growth along their
anterior margins.

CARDINAL AREA. Omitting for the present the Strophomenide
and Orthidz, the initial state of the ventral cardinal area for all other forms
is a broad triangular opening beneath the beak, with simple sharp margins.
This condition is never passed by Merdstella rectzrostra, which shows a
uniform, open, triangular area in every period of growth.

A farther advanced state of progress initiates the deltidial plates, which
first appear as narrow laminz along the sides of the area. The areal de-
velopment of Spzrzfer crispus, Rhynchonella neglecta, and R. acinus, ceases
at this point.

In the next stadium, the further growth of -the deltidial plates along ~
their free edges gives them a triangular form, and they tend to narrow the
limits of the opening and define the peduncular foramen. Spzrz/er nzagar-
ensis and Rhynchonella whitit represent species which are arrested at this

- period.

The completed growth shows the deltidial plates uniting by symphysis
along a median line, and inclosing near the apex of the area a more or less
circular pedicle-perforation. Rhynchotreta cuneata, Whitfeldia maria,
Retzia evax, etc., after passing through all the earlier conditions, reach this
limit of development.

The results of senile and extravagant growth often obliterate or de-

88 MEMOIRS OF THE STATE MUSEUM

generate the normal deltidial advancement, the plates becoming thickened
and their features obscured, while in some species processes are given off,
as in a number of the Mesozoic Rhynchonellide.

The cardinal area of the Strophomenidz in its early phase shows a
small pedicle-sheath for the ventral valve and a narrow grooved process
under the beak of the dorsal. The perforation for the passage of the
‘peduncle does not materially increase in size with the growth of the shell
and often is obliterated, while the dorsal callosity usually reaches a consid-
erable development.

Additional evidence of the degeneracy of the pedicle is afforded by
many species of other genera, which have a calcareous attachment to for-
eign objects at the apex of the ventral valve, the pedicle, therefore, becom-
ing functionally obsolete.

Observations having some analogy with the facts here presented have
been made, in a very restricted sense and usually incidentally, by various
authors. Our own results, though derived from the species of a single
fauna, must not be given too limited an application, for they involve nearly
every important family of paleozoic articulate brachiopods, and we may
tentatively assume that, as a rule, the essential features of variation ob-
served in any member of a genus will hold good of the other members.
In regard to the development of the characters of the pedicle-passage, ‘z. ¢.
the deltidial plates and the foramen, there is good reason to regard the
process as substantially identical in all the genera represented, making the
necessary allowance for the peculiar variation seen in the Strophomenide,
which may not, however, prove it an exception to the general statement.

The various terms which have sometimes been applied to the condition
of the deltidium in the rostrate genera, as de/tzdiwm amplectens, when the
foramen is entirely surrounded by the plates, as in various Mesozoic Rhyn-
chonelle (but in no paleozoic species of which we are aware), de/tzdium sec-
tans, when the plates bound the foramen only on the lower side, the upper

a ——— os gel, | eet em

SILURIAN BRACHIOPODA | 89

side encroaching on the substance of the umbo, as in 7erebratula, Merzs-
tina, etc., deltidium discretum, when the plates do not come into contact, as
in Zerebratella, some species of Rhynchonella, etc., must be regarded as
having no further significance than to express the existing condition of the
foramen and deltidium in any given sfeczmen, that is, as indicating a stage
of development, not necessarily a generic or even specific character.

The observations of M. Eucene Drstonccuamps upon these features
are of much value, and in most respects, as far as carried out upon related
forms, are in harmony with our own (Note sur le développement du deltid-
ium chez les brachiopodes articulés: Bull. Soc. Géol. France, 2° Ser. T. xix,
pp. 409-413, pl. ix, 1862), but with his conclusions, we are forced to
differ in some respects. The investigations referred to were made upon
one (or more) Mesozoic species of 7erebratula, Rhynchonella, and Spirifer-
ina, specific designations not given. The illustrations of Zerebratula (figs.
1a, 6, c, and column A, a, f, y, 6) show in effect the characters seen in
Meristina nitida, Whitfeldia marza, and others; those of Rhynchonella
early stages of similar character, resulting in a de/tzdium amplectens, such,
as just observed, we have failed to discover in paleozoic Rhynchonelle.

In Spiriferina, according to the author, the deltidium is produced by
the gradual development of a single plate in the apex of the triangular
opening, increasing downward with age, a very distinct mode of formation
from all the others, and open to verification in the species described by M.
Destonccnamps, as his figures make no allowance for a pedicle-sinus or
perforation, a feature, though not of frequent occurrence in the Spiriferide,
yet one necessary to account for.

We have examined specimens of Spzriferzna pinguzs, Deslongchamps,
S. rostrata, Schlotheim, and S. wadcott?, Sowerby, and find that these
species, at least, develop triangular deltidial plates. Those in Spzer¢ferina
pinguts and S. walcott2 are comparable with the same parts in Sperzfer

perlamellosus and S. cumberlandie,and their form and mode of growth
¥

go MEMOIRS OF THE STATE MUSEUM

-

are expressed by the outlines 2”, 3”, on page 79, and is further shown in
the accompanying illustration. Additional ! z

growth causes the plates to unite along the /\ IN
median line, obliterating the partially formed

pedicle-perforation, and subsequent increment , , ‘
can naturally only take place along their rower /\ A TIN
free edges, Fic. 4. DELTIDIAL DEVELOPMENT IN

Our remarks on Sperifer radiatus and S. Dk Se aon
crispus indicate that the development of the Se a ae
plates in this member of the same family is quite in harmony with the
process as seen in the rostrate forms generally.

The following is the summarization of DEsLonccHAmps’s conclusions as
given by himself :

(1) The deltidium is one of the most important features in the articu-
lated brachiopods.

(2) As far as Jurassic species are concerned, the deltidium may suffice
to characterize the families.

(3) In the various stages of development of this part, the aspect of the
shell is entirely changed.

(4) The deltidium appears under three important modifications: A.
development below the-peduncular arm, characterizing the Terebratulide,
C. development above the peduncle, Spiriferide, B. mixed development,
surrounding the peduncle, Rhynchonellide.

(5) The stage at which the development is arrested or the exuberance
of development may suffice to characterize sections under the families.

It has just been shown that conclusions 2, 4, and 5 are not capable of
the extended application which he has given them.

A preceding remark, that the course of development of the deltid-
ial characters throughout the genera here discussed, may be considered as

fundamentally uniform, calls for explanation in its application to Or¢hzs and

SILURIAN BRACHIOPODA gI

the Strophomenidz. In the latter forms, we have seen that the remarkable
development of the pedicle-sheath is primary, and is invariably more or less
atrophied with age, and probably functionally inactive at maturity. Hence
the retention of this sheath in any species at maturity is the perdurance of
what must serve as an embryonic character within the limits of this family.
It cannot escape observation that the pedicle-sheath is in analogy with the
entire rostrate umbo of the ventral valve in the Rhynchonellz, Terebra-
tulz, etc., as a specialized extension of the valve for the protrusion of the
“pedicle (compare the extreme development of the umbo in the genus
Terebrivostra). That these parts are also homologues, it is difficult to prove,
on account of the pedicle-sheath becoming more degenerate as maturity
approaches, but, assuming this homology, we may regard the sheath and its
gradual disappearance, as an indication of degeneracy in the family, the
presence of the sheath pointing toward a derivation from the rostrate type.
The atrophy of an organ so highly specialized as the sheath, is, aside
from any consideration of relationship to other groups of the brachiopods, it-
self confirmatory of such degeneracy. Furthermore, it will be noticed, that
there is, throughout these Strophomenoids, an inclination, as mature growth
comes on, toward the simple triangular pedicle-apertures in Orthzs. The
disappearance of the pedicle-sheath leaves the aperture of the ventral
valve essentially free, as seen in Strophomena rhombotdalis and Streptorhyn-
chus subplanum, while the aperture of the dorsal valve is filling parz passu
with a callosity. In other words, the structure of these parts is actually
degenerating toward maturity, to that of Or¢hzs, which is the simplest, least
differentiated condition among the articulated brachiopods, and serves to
fortify the position of the genus at the base of the entire series. In Orthzs,
the pedicle-apertures on both valves are of the same size in early growth,
and have undoubtedly acted together as a single opening, through which
the fleshy arm was protruded as much on one side as the other, a fact indi-
cative of an extreme lack of differentiation in the two valves in the articu-

92 MEMOIRS OF THE STATE MUSEUM

late species, but agreeing closely with some of the inarticulate genera, e. g.
Lingula, Leptobolus, Obolus. The specialization which accompanies subse-
quent growth confines the pedicle more closely to the ventral aperture, and,
as a result, the dorsal aperture is gradually filled by a callosity. Thus,
also, the Strophomenide ;-but Streptorhynchus subplanum shows at maturity
what has not yet appeared in Or¢hzs, viz., the initiation of deltidial plates,
in conformity with the general course of development of the cardinal
features observed in other families.

It is not well in this place to go beyond the scope of this work,
and the species of Strophomenide here discussed for facts confirmatory
of our observations. It may be remarked, that the stage at which the
development of the deltidial features has been arrested at maturity in this
family varies with the species, not with the genus. When every trace of
these features is obliterated, as is usual in Strophodonta, a slight abrasion
of the apical substance of the shell will often show a trace of the obsolete
pedicle-tube. At times, in the same genus, this is retained at maturity as
an external feature, and in such a case is usually accompanied by some
indication of the subapical sheath. In both Strophodonta and Streptor-
hynchus (especially of the later paleozoic faunas), the cavity of the pedicle-
sheath, if it be retained in any form, at maturity, has been filled by the
deposition of calcareous matter about the compound cardinal processes of
the opposite valve, and thus wholly diverted from its original function.

In conclusion, it is to be observed, that of recent species of brachiopods
a very great number show an incomplete development of the deltidial
plates at maturity. Such is Rhynchonella to a large degree; also Czséedla,
Argiope, Kraussina, Terebratella, and Magasella ; and we may assume the
structural degeneracy which is thus indicated, to be the natural concomitant
of the secular decline of the entire class.

It is not improbable that from an early form related to the genus
Orthis, phylogenetic development tended in two main channels. One

TT aes

“> il

SILURIAN BRACHIOPODA . 93

leading through Strophomena, Scenidium, Orthisina, Leptena, Chonetes,
Productus, and Strophalosza, and the other in the direction of Rhyuchonella,
Spirifer, Atrypa, Retzia, and Terebratula.

INTERNAJ. APPARATUS. Our observations upon the development of
the brachial supports are limited to the species Retzia evax. Here it is
found that the number of revolutions of the spiral ribbon increases with
age, but we are not prepared to say what the inceptive condition of this
apparatus may have been. In the early stage represented on page 60,
where the ribbon has completed two revolutions, the supports must have
been exceedingly tenuous and delicate, for they can be traced in the crystal-
line or muddy filling of the shell, only by extremely faint lines, composed of
minute dots of pyrite. As observed under the discussion of these features,
the character or actual existence of the loop connecting the spirals was not
established, but it is developed, with all normal characters, in a shell 4 mm.
in length, where the ribbon makes four revolutions.

It has been shown by Morsse,* that in Zerebratulina septentrionalts, the
loop (i. e. the entire brachial support) begins by the development of two
acute processes from the lower moiety of the dental plate, which assume
the character of crura, eventually meeting and coalescing on the dorsal side,
forming the completed loop at an early stage, the ventral horns of the loop
never uniting. The simple nature of the support in these shells precludes
the possibility of the continued growth which obtains in the more compli-
cated apparatus of the spiriferous species. The inception of the brachial
support was observed by Morse in an individual 1 mm. in length, but the
lateral processes are not conspicuously developed until a length of 3 mm. is
attained, and they have not united at a length of 4mm. It is therefore
possible, from these data, that Retzza evax does not have the loop ¢om-
pleted at so early an age as that indicated by a length of 2.5 mm.

* Memoirs of the Boston Society of Natural History, Vol. ii; On the Early Stages of Tere-
bratulina septentrionalis, pl. 2, figs 48-55. 1869.

94 MEMOIRS OF THE STATE MUSEUM

The observations by Morse are corroborated by those of Dat1* on
Terebratula cubensis.

SuRFACE ORNAMENTS. Nearly all the observations upon initial shells
or upon that portion situated at the apex of the beak of more advanced
stages and representing the initial shell, seem to warrant the assertion
that the surface ornaments do not appear until the second or a later period
is reached in the development of the shell.

For the plicate species, nearly the full number of plications appear
simultaneously, as in Rhynchonella indianensis, R. acinus, and Rhynchotreta
cuneata, or they are introduced in pairs, as upon Rhynchonella whitt2,
? R. neglecta, and Retzta sobrina.

The strie of Strophomena rhombotdalis are developed to the full
capacity of the marginal area as soon as the first growth-line is completed.
The number is increased in three distinct manners, (a) by division,
(b) by intercalation, and (c) by addition at the cardinal angles. Some
species present all three of these, while others add to their striz or
plications by any one or two of the modes.

The concentric ornamentation in such species as Spirzfer crispus and
Streptorhynchus subplanum appears early in the growth of the embryo, and
continues to be repeated without variation, except in Strophomena rhom-
boidalis and allied forms, which develop, during the last stage of growth,
a geniculated curtain without concentric undulations.

VARIETIES AND ABNORMALITIES. Varieties usually begin to express
themselves early in the development of the shell, and the divergence
from the normal form rapidly increases as maturity approaches. Several
of the species represented by abundant material, are readily separable
into three distinct groups of ‘forms, (@) dong form, (6) normal form, and:
(c) broad form. The history of each may be clearly traced, and they

* Proceedings of the Academy of Natural Sciences of Philadelphia, 1877, pt. ii; Report on
the Brachiopoda of Alaska and the adjacent Shores of North-west America, p. 155.

SILURIAN BRACHIOPODA 95

usually are found to unite with the line of the normal form (4), several
removes from the initial member of the series. Sometimes the varieties
do not reach the adult dimensions of the normal shell and may be con-
sidered as varietal dwarfs.

The representation of varietal and of certain abnormal conditions by
complete series of fossil specimens shows, that in these directions, there
was a distinct developmental tendency, or genetic impulse, independent of
normal growth.

SENILITY igalways expressed by the thickening of the shell which takes
place after the individual reaches adult size. The thickening may involve
the whole interior of the valves, producing a truncate appearance at the
margins, or it may take place by frequent interrupted growth along the
margins, giving to this portion a varicose aspect. As a result of this senile
growth, the vertical diameter of the shell is increased, and the beaks
involuted, so that they are often so closely appressed as to conceal the
cardinal area and truncate the ventral beak, and in addition, the margins
of the valves lose the characteristic ornamentation of the species and cor-
respond to the clinologic and nostologic stages as defined by Mr. Hyarr.*

Abnormalities frequently find an explanation in some pathological or
accidental conditions which become instituted at any period in the life of
the animal, and leave their impress on the shell. The functional failure
of a developing organ may cause the parts to revert to an embryonal type,
and although it is difficult to apply this statement to the shelly covering
of the animal, yet this condition is sometimes found. The specimen of
Rhynchonella neglecta described on page 37 is an instance of this
kind. Another abnormal variation is noticed in certain individuals which
preserve the larval features of the shell long after it has passed the early
stages, and when, in many cases, it has reached the full adult dimensions.

* Proceedings of the Boston Society of Natural History, Vol. xxiii, 1888; Values in Classi-
fication of the Stages of Growth and Decline, with propositions for a New Nomenclature.

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PLATE I

CRANIA SILURIANA, Hall
Page 13 5

. The youngest individual observed; having a height of 1 mm, and a width across the base of 1.5mm, The elevation of

the shell is in strong contrast to that of the mature form.

. A mature individual attached to a shell of P/atystoma (28th Rept. N. Y. State Mus. Nat. Hist., pl. 21, fig. 5).

ORTHIS ELEGANTULA, Dalman

Page 13
See Plate viii

. Dorsal view of the youngest embryo observed, its length being.5 mm.; its width.75 mm. The median sinus has already

formed, and three pairs of plications have appeared, of which the middle pair is evidently the oldest.

. Outline profile of the same; showing the slightly greater convexity of the ventral valve.

A larger example viewed from the dorsal side, its length being 1 mm., its width 1.5mm. The plications have now in-
creased to the number of six pairs, one of which has appeared between the median plications seen in fig. 3.

Ventral view of an individual in about the same stage of development; showing a strong median plication corresponding
to the dorsal sinus, and five pairs of lateral plications.

Cardinal view of an individual in the growth-stage represented by fig. 5. The valves have nearly the same convexity,
while the width of the cardinal area and the size of the pedicle-passage are the same for each, The latter is seen to be
quite unobstructed and without further differentiation that a slight thickening of the margins.

. Outline profile of the same.
. Cardinal view of an individual which has reached a size of 3x3mm. Here is apparent a change in the development of

the cardinal area and foramen of the valves. The primary indication of the callosity or cardinal process is in the

apex of the dorsal openi The difference in thé convexity « of the valves al ceably increased.
Outline profilé of the samé. _< Pe i aT tage

. Cardinal view at a size of 5x5 mm. The ventral beak has become strongly incurved, and the cardinal process is now

subdivided into three parts. The cross-lines, representing the natural size of the specimen, are too short.

. Outline profile of the same.

Cardinal view at a size of 12x11 mm. The ventral valve and area have become greatly curved, and the dorsal aperture
is quite filled by the tripartite cardinal process.

. Outline profile of the same.
. Cardinal view of a large adult, size 18x 18.5 mm, ‘The areas are closely appressed, and the dorsal aperture is wholly filled

by the cardinal process, the central portion of which extends into the aperture of the other valve.

. Outline profile of the same,

. Cardinal view of a normal adult; natural size.
. Profile of the same (of. c7#. pl. 21, figs. 17, 14).
. Dorsal view of a small adult.

. Ventral view of the same (of. c#t., figs. 12, 11).

ORTHIS HYBRIDA, Sowerby
Page 17

Fig. 13. Cardinal view of a very young individual having a length of 1 mm. anda width of 1.5 mm. The valves are nearly

equiconvex, the area and apertures as in the earlier stages of O. elegantula,

Fig. 134. Outline profile of the same.
Fig. 14. Cardinal view of a somewhat gibbous example measuring 10x8mm, The relatively short areas are about equally de-

Fig. 142.

veloped, and the cardinal callosity of the dorsal valve has already filled the dorsal aperture.

y

Outline profile of the same,

Fig. 15. Cardinal view of a normal adult 12x 10 mm.; showing the short area and the projection of the cardinal process into the

ventral aperture.

Fig. 154, Outline profile, showing the incurvature of the areas.
Fig. 16. Dorsal view of a large adult (of. c7t. fig. 20).

Fig. 17.
Fig. 18.
Fig. 182.

Profile of a normal adult (of. eft. fig. 21).
Dorsal view of a small, gibbous example, showing strong varices. Enlarged to two diameters.
Profile of the same.

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PLATE II

STROPHOMENA RHOMBOIDALIS, Wilckens
Page 18

Fig. 1. Ventral view of the youngest shell observed, its length being 1.2mm. The aperture of the embryonal pedicle-sheath
is very conspicuous, and its margins very thick. The surface shows a faint median depression, indications of two con-
centric growth-lines, and outside the latter of these, obscure traces of plications.

Fig. 1a. Outline profile of the same, showing the prominence of the sheath,

Fig. 2. Ventral view of an individual with alength of 2mm. The aperture of the pedicle-sheath is relatively somewhat dimin-
ished in size, its margins have become thinner, and the radiating plications numerous and sharply defined.

Fig. 2a. Outline profile of the same, indicating diminution in the prominence of the sheath.

Fig. 3. Ventral view of an individual having a length of 4 mm.; showing the increase in the number of plications, the appearance
of ‘ous ic undulations and striae, and the narrowing pedicle-aperture.

Fig. 3a. Outline profile of the same, showing the concentric undulations and the diminishing pedicle-sheath.

Fig. 4. Ventral view of a normal adult, natural size, having a length of 28 mm.; showing the characters of maturity.

Fig. 42. Profile of another individual of full-growth; showing the anterior geniculation and the length of the anterior slope or
curtain. (28th Rept. N. Y. State Mus. Nat. Hist. pl. 22, figs. 6, 7.)

Fig. 5. Cardinal view of the specimen represented by fig. 1.

Fig. 6. Similar view of the specimen represented by fig. 2.

Fig. 7. Similar view of an individual 2.65 mm. in length

Fig. 8. Similar view of the specimen represented by fig. 3.

Fig. 9. Similar view of an individual 9 mm, in length, These cardinal views are drawn with the same degree of enlargement,
and show the gradual diminution in height and in diameter of aperture in the pedicle-sheath, and the increasing de-
velopment of the grooved callosity on the dorsal valve.

Fig. 10. Cardinal view of a normal adult, natural size; showing the great size of the grooved callosity, and the cacal opening,

Pig The ca abarea efron tx Ref cede Seams ile nf wi fe podrominnt, eer

ens at its base, the Fane grooved dorsal

Fig. 12. Prag cardinal area shown in fig. 6 (length 2’ mm.), enlarged to the size of fig. 11; showing the depression of the sheath,
the narrowing of the cardinal area, and the increasing aperture between the sheath and callosity.

Fig. 13. The pedicle-area of a mature individual, x2, The sheath is now wholly absorbed, the sole trace of it being seen in the
caecal foramen, surrounded by the umbonal portion of the shell. The callosity is strongly developed, but not suffi-
ciently to close the gap between it and the opposite valve, thus leaving a passage between the valves and along the
dorsal groove.

STREPTORHYNCHUS SUBPLANUM, Conrad

Page 23
See Plate viii

Fig. 14. Ventral view of the smallest individual observed, having a length of 2.2mm. Both primary and secondary plications
and concentric growth-lines have already appeared, indicating the very early ee of these characters

Fig. 14a. Outline profile of the same; showing the convexity of the valves.

Fig. 15. A normal adult, dorsal view.

Fig. rsa. The same in profile (of. c##. pl. 21, figs. 30, 31).

Fig. 16. Cardinal view of speci somewhat larger than that represented in fig. 14. The ventral valve bears a small pedicle-
sheath, the dorsal, the inception of a cardinal process or callosity, while between the two is a broad opening, which
serves to indicate that, at this early age, the pedicle-sheath had ceased its function,

Fig. 17. Cardinal view of an individual slightly below normal full growth, but with essentially mature characters.

Fig. 18. The pedicle-area of the specimen represented in fig. 16,

Fig. 19. Pedicle-area of a shell having a length of 4 mm. At this stage of growth the sheath has relatively diminished in size,
while the dorsal callosity has increased and shows a median groove on its inner edge. Deltidial plates have also be-

+ gun to develop along the margins of the ventral aperture.

Fig. 20, Pedicle-area of the specimen represented in fig. 17, The sheath is now atrophied and altogether obsolete, the dorsal
callosity is very large, nearly filling the aperture between the valves, and the deltidial plates have attained the max-
imum development observed in the Strophomenida.

The last three figures have the same degree of enlargement.

= RACEHIOPODA ‘

MEM.N.Y.STATE MUS. PLATE XZ,

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PLATE III :

STROPHONELLA STRIATA, Hall

Page 25
Ventral view of the incipient shell of the series (length 2.25 mm.); showing the opening of the pedicle-sheath, and the
mary surface plications ;

Fig. 1a, Outline profile of the same; showing the complete convexity of the ventral valve, and essentially confarmable concavity

Fig. 2.
i

3.

ig. 4
&
me &

Fig. 9.

Fig. 9a.
Fig. 94.
oe
Fig. 10a.
Fig. 106.

Fig. 11.
Fig, 12,
Fig. 13.
Fig. 134.

Fig. 14.
Fig. r4a.
Fig. 15.
Fig, 16.
Fig. 162.

Fig. 17.

Fig. 17a.
Fig. 18.

Fig. 182.
re: 19.

Fig. 20.

Fig. 21.

Fig. 22.

Fig. 23.

Fi,

ig. 26.
Fig. 26a.
Pig. or.

Fig. 274.
Fig. 28.

Fig, 282.

Big. age.
. 24a.
Fig, 25.

Pe- 25a.

of the dorsal valve.
Ventral valve of a_ normal adult; showing the umbonal convexity of the shell and general concavity over the pallial
region. (a8th Rept. N. Y, State Mus. Nat. Hist., pl. 23, fig. “4
Outline profile of the same; showing the reversal in convexity from the embryo condition.
Pedicle-area of the imen represented in fig. 1 (length 2.25 mm.); showing the well-developed, slightly exsert sheath
and the obscure dorsal callosity.
Pedicle-area of an individual, 2.5 mm. in le in which the sheath is extravagantly exsert.
Pedicle-area in an example about 6 mm. in | :
Pedicle-area, when a length of 8 mm. has been attained.
Pedicle-area in a shell measuring 13 mm. in length. . ~
Pedicle-area in a normal adult measuring 17 mm. in le u
Figs. 3-8 have been drawn to the same scale, and show the successive phases in the development of eating is

characters. The sheath ceases its function as a pedicle-passage before maturity is attained, though its
relative size, while the dorsal callosity, which in the earlier stages is grooved and largely enveloped b sheath,
is eventually separated from the sheath by a narrow aperture, and its surface becomes uninterrupted.

STREPTIS WALDRONENSIS, Miller and Dyer 5

Page 30 ,
Dorsal view of a young individual having wn a of 3mm. The shell is nearly symmetrical, and shows an open triangu-
lar deltidi ding in a subcircul ory ‘oramen.
Ventral view of the same; showing the apical foramen.
Outline profile of the same.
Cardinal view of an adult specimen; showing the asymmetrical shell.
Dorsal view of the same. :
Ventral view of the same. (11th Ann. Rept. State Geol. Indiana, pl. 27, figs. 21, 19, 20.)

EICHWALDIA RETICULATA, Hall r

Page 3:
A young individual havin x length and width of 3 mm.; showing the subcircular outline and undefined median fold.
2
aH

fold
Axial jon ofya lar, ya sat in egeing oo as of se ia a and ho: fa ternal ventral
slate and dorsal Spi ay P ‘ys Yi “y1 i > et 2 T fon.) ¢
Cardinal view of the same; showing the bare umbonal area and the lines of lateral attachment of the internal plate, x2.
(Figs. 13, 13¢ from the 28th Rept. N. Y. State Mus. Nat. Hist., pl. 26, figs. 53, 54.)

ANASTROPHIA INTERNASCENS, Hall

Page 32
Ventral view of the youngest specimen observed (length 2 mm.),
Outline profile of the same; showing the elevation of the ventral beak and cardinal area.
Dorsal view of a large adult.
Ventral view of an average adult; showing the veteres dorsal valve.
Profile of the same; showing the relative convexity of the valves.
(Figs. 15-16a from the 28th Rept. N. Y. State Mus. Nat. Hist., pl. 26, figs. 49, 45, 44.)

RHYNCHONELLA INDIANENSIS, Hall

Page 42

The earliest stage of growth observed, the shell having alength of .65 mm. The characters are essentially primitive;
the surface is without plications, the foramen —— and devoid of plates, or even marginal thickening.

Outline profile of the same; showing the elevation of the ventral umbo. .

A later stage of growth in which the shell has a length of 1.5 mm. With the formation of the first growth-line, a number
of faint plications have appeared, and the margins of the foramen have become thickened.

Outline profile of the same. *

A young = with a length of 6 mm.; showing the inception of the median fold, (11th Ann. Rept. Geol. Surv. Ind.,
pl. 27, fig. 6.)

_— view of a larger {zemole. natural size, having two plications on the fold, and abnormal in the absence of all lateral

ications (of. c7t., fig. 5).

Dorsal view of a small, conattelhy mature example, with two ag peri on the fold (of. cét., fig: 4).

Dorsal view of anadult with three plications onthe fold. (28th Rept. N. ¥Y, State Mus. Nat. Hist., pl. 26, fig. 13.)

Front view of an adult with four plications on the fold (of. c#t., fig. 22).

view of a similar individual.

Profile of the same (of. cit. figs. 15, 19).

An pny mag the cardinal area in an individual with a length of 1.5 mm.; showing essentially the primitive charac-
ters seen in ~ 17

Outline profile of the same. ‘

The condition of the ventral cardinal area and foramen in an individual 3.5 mm, in length. The margins of the foramen
it thickened by the inception of the deltidial plates and the aperture is seen to encroach upon the apical portion of the

Ss
Outline profile of the same,
The cardinal features in an individual of 7 mm. length. The advance upon the last stage is chiefly in rapid development
of the deltidial plates, which have narrowed the opening, slightly constricting it near the now incurved umbo.
Outline profile of the same.
The cardinal features in an adult, having a length of 1omm, The deltidial plates have united at their base, forming an

elongate oval aperture, encroaching upon the umbo. The species does not pass this stage of development.
Outline profile of the same.

Sr ACP IOLFOoDsA..

MEM.N.Y. STATE MUS...

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Fig. 6.
Fig. 6a.
Fig. 7.
Fig. 7a.
Fig. 8.
Fig. 82.
Fig. 3

Fig. ~,

. Dorsal view of the same.

: Outline profile of the same
. Cardinal area in an individ

PLATE IV

RHYNCHONELLA WHITII, Hall
Page 39

. The earliest observed stage of growth, the shell measurin; A Beg in length by 2 mm. in width. The deitidial plates
angu

have already begun to form along the edges of the
dorsal valve.
Outline profile of the same.
Dorsal view of a normal adult having two plications on the fold; natural size.
Profile of the same.
Front vjew of the same. (28th Rept. N. Y. State Mus. Nat. Hist., pl. 26, figs. 23, 26, 25.)
fa eer gs features of a young example with a length of 3.25 mm.; showing the inception of the deitidial plates.
je of the same.
Cardinal features of an individual which has attained a length of 6mm. The umbo has become incurved and the devel-
opment of the plates is well advanced, but it is arr at this stage, the foramen not becoming inclosed at maturity
Outline profile of the same.

RHYNCHONELLA NEGLECTA, Hall

Page 37

Dorsal view of the youngest shell observed, having a length of .75 mm., a width of .5 mm. ity the foramen is open
and without evidence of thickened margins, four fine plications have already ap on the dorsal valve,

Outline profile of the same. .

Dorsal view of an individual with a length of 2.25mm. The deltidial plates are in an incipient condition, and the plica-
tions of the surface have considerably increased.

Outline profile of the same.

Dorsal view of a ple; I size.

The same in profile. (28th Rept. N. Y. State Mus, Nat. Hist., pl. 26, figs. 3, 5.)

A somewhat abnormal adult, having three plications on the fold and the plicati
Rept. Geol. Sur. Ind , pl. 27, fig. 3.)

foramen, and four pairs of plications are visible on the

obsol » x2. (11th Ann,

RHYNCHONELLA ACINUS, Hall

Page 35
Ventral view of a young shell in a secondary stage of growth; showing a single growth-line and the embryonal median

sinus iswell di J

ridge.

% Dorsal® view of the same. The foramen is slightly narrowed, but without ye aes t or ES a the embryonal
yy, Tc] iz f : ‘

. Outiine/profile of the same. lI) AOTLTAYALTTA. ;

Ventral view of a specimen 3.25 mm. in length.
The mature fold and sinus have not yet begun to develop; the foramen shows increased con-
striction at its base, and slightly thickened margins.

. Outline profile of the same.
. Dorsal view of an average adult, x 4.
. Profile of the same.

Front view of the same; showing the elevation of the median fold.

RHYNCHOTRETA CUNEATA (Dalman), var. AMERICANA, Hall
Page 47
See Plate viii

Dorsal view of ashell resenting the earliest e of wth observed (1 h x.5mm.). The foramen is widely tri-
angular and iapbalracted; the purtace is poh with » all the plications ob ata, and the subcircular valve
a broad median depression.

. Outline profile of the same.
. Similar view of a shell3 mm. in length. The outline has become elongate, the foramen narrowed and its margins thickened.
. Outline profile of the sam

ec.
jew of a normal adult individual, natural size; be ed the elevated beak and broad plications.

. Profile of the same, showing the ele vation of the dorsal fold.
. Front view of the same (28th Rept. N. Y. State Mus. Nat. Hist., pl. 25, figs. 31, 34, 36.) j
. A series of outlines of the anterior margin; showing the embryonal sinus or depression in the dorsal valve in young

forms, reaching its maximum in individuals having a } of 4.5 and becoming obsolete, or the line of junction
straight, in specimens 7 mm. in length. Outline 6 shows the inception of the dorsal fold, In 8 the four median pli-
cations are distinctly elevated ; 9 is from a normal full wn individual and ro represents the maximum elevation of
c bd ger be outlines, except in 9 and 10, are anlargot to the diameter of a fully developed specimen.
ar area 0! . 12.

; Satine pox of the same.

area of fig. 13.

ual 4.5 mm. in length; showing the considerably advanced development of the deltidial plates
the of Hi én on the apex of the valve. y

an
. Outline profile of the same

Cardinal area at a length of 5 mm.

‘9a. Outline profile of the same.

Cardinal area at a length of 7 mm.

. Outline profile of the same.
. Cardinal area in an individual which has attained a1 of 12 mm. The plates are united for nearly one-half their

length, and slightly bent outward about the base of foramen.

. Outline profile of the same.
G

nal area at maturity; showing a stronger flexion of the plates below the foramen, and a subcircular foramen, en-
ing upon the apex of the shell.

croach
. Outline profile of the same.

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MEM.N.Y. STATE MUS. PLATE sv.

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PLATE V

: RETZIA EVAX, Hall

Page 55
See Plate viii

Fig. 1+. Dorsal view of the youngest individual observed, having a length of 1 mm. anda width of .8mm. The ventral umbo
is erect, the foramen triangular and without deltidial plates; the surface bears two faint lines of growth, and outside the
second of these, are three fine plications on each 'side of a median sinus.

Fig. 1a, Outline profile of the same; showing the very shallow valves.

Fig. 2. Dorsal view of an average adult, showing the characters of advanced growth.

Fig. 2a. Profile of the same. (28th Rept. N. ¥. State Mus. Nat. Hist., pl. 25, figs. 14, 18.)

Fig. 3. Dorsal view of an immature individual, 5 mm. in length, in which the plications on the earlier portion of the shell end
abruptly at a growth-line, from there outward the surface characters being altogether primitive.

Fig. 34. Outline profile of the same.

Fig. 4. The cardinal area in its earliest observed condition ; enlarged from fig. 1.

Fig. 5. A later stage of growth in the cardinal area, the deltidial plates having a considerable development.

Fig. 6. A still later condition of this area, in which the plates have united, enclosing the foramen.

Fig. 7. The cardinal portions of an individual with an unusually elevated ventral umbo; showing also an advance in growth

from the condition represented in fig. 6.
Fig. 8. The character of the cardinal area in a normal adult of about the size represented in fig. 2. The foramen has become

circular and the ventral umbo so incurved as to conceal the deltidial plates.
Fig. 9. A small obese example in which the foramen is almost wholly concealed.

RETZIA SOBRINA, sp. nN.

Page 61
The youngest shell observed, having a length of 2 mm. anda widthof 1.6mm. The shell already bears two plications
on each side 6f the median s, and uch fainter ‘ize inthe Sinus itself." 7 ~

Fig. 10a. Pv. 333 oF A a 2 . rt rt i] raat TT ron A a It 4 4

Fig. 11. Dorsal view of an adult specimen, somewhat above the average size, and enlarged to 114 diameters; showing the normal
features of maturity.

Fig. 11a. Profile of the same.

Fig. 114. Ventral view of the same individual; natural size.

Fig. 12. Transverse section of an individual; showing the spiral ribbon and the number of volutions.

Fig. 13. The character of the pedicle-passage in the youngest example (fig. 10). The deltidial plates are absent, but the margins
of the foramen are thickened.

Fig. 134. Outline profile of the same.

Fig. 14. A later stage of growth, in which the plates are considerably developed. .
Fig. 14a. Outline profile of the same.
Fig. 15. A stage of growth in which the f has b ircular and apical. The plates are slightly flexed along two oblique

lines which converge toward the base of the area.

Fig. 15a, Outline profile of the same.

Fig. 16. The mature condition of the pedicle-area, the deltidial plates being somewhat concealed by the incurvature of the ven-
tral umbo.

Fig. 16a. Outline profile of the same.
The drawings 13-16a are drawn to a scale.

CCELOSPIRA DISPARILIS, Hall -

Page 64

Fig. 17. A young individual 2.5 mm. in length; showing surface and pedicle characters, essentially as at maturity. The sinal
plications apparent at this early stage, appear, in the usual adult form, as the broad median fold.

Fig. 17a. Outline profile of the same; showing the erect beak and shallow valves.

Fig. 18. A young individual; showing asymmetry in the development of the median fold, and a greater number of lateral plica-
tions than in the normal adult (enlarged).

Fig. 19. Dorsal view of a normal adult.

Fig. 19a. Ventral view of the same.

Fig. 194. Profile of the same. All are enlarged to two diameters. (28th Rept. N. Y. State Mus. Nat. Hist., pl. 25, figs. 39-41.)

Figs. 20-23. Enlarged views of the umbonal region of the ventral valve; showing the variations of form and position in the foramen
as observed among specimens which differ but little in actual size.

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PLATE VI

SPIRIFER BICOSTATUS, var. PETILUS, Hall
Page 75

Figs. 1, r@, The youngest example observed, and outline profile of the same.

Fig. 2.
Fig. 3.

Fig. 4

Fig:
Fig. 5.

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Fig. 8.

Dorsal view of a large specimen.

Cardinal view of the same, x3. (11th Rept. State Geol. Ind., pl. 27, figs. 8, 9.)

SPIRIFER CRISPUS, va7. SIMPLEX, Hall

Page 75
Dorsal view of the youngest growth-stage observed. Within the single growth-line, the smooth subcircular initial shell
is visible, but outside this, the shell has developed the median fold and the surface fimbriz.

. Outline profile of the same.

Dorsa) view of a mature individual, x 2.
Profile of the same, (28th Ann. Rept. N. Y. State Mus. Nat. Hist., pl. 24, figs. 1, 4.)

SPIRIFER CRISPUS, Hisinger
Page 75

. Dorsal view of a young form, in which the deltidial plates are in an ong condition.
. Outline profile of the same.

. Dorsal view of a mature specimen.

7a.

Profile of the same, (28th Ann. Rept. N, Y. State Mus. Nat. Hist., pl. 24, figs. 8, 12.)

SPIRIFER NIAGARENSIS, Hall
Page 80
The cardinal area of a ventral valve, from Lockport, New York. The deltidial plates are in the third stage of develop-

sgh tt ia pe ihn hei ae Sara a
SPIRIFER RADIATUS, Sowerby
Page 77

. A cardinal view; show oe a similar stage of development of the deltidium; from an individual which has not attained

normal full-growth.

. The youngest growth-stage observed.
. Outline profile of the same.

. Dorsal view of a normal adult.

. Profile of the same,

(28th Ann. Rept. N. Y. State Mus. Nat. Hist., pl. 24, figs. mes

ATRYPA RETICULARIS, Linnzus
Page 51

. The incipient shell, having a length of 2.2mm. The deltidial plates have already begun to form, and the shell has de-

veloped two growth-lines.

. Outline profile of the same.

. Ventral view of a full-grown individual.

. Profile of the same. (28th Ann. Rept. N. Y. State Mus. Nat. Hist., pl. 25, figs. 46, 47.)

. A series of outlines of the anterior margin of the valves; showing the rapid increase in plications and the reversion from

the embryo fold and sinus to the sinus and fold of maturity.
The deltidial characters of the specimen represented in figure 12.

. Outline profile of the same.

. Similar view of a specimen measuring 3 x 3 mm.

. Outline profile of the same.

. Similar view of a specimen measuring 3.5 x 4 mm.,

. Outline profile of the same.

. Similar view of a specimen measuring 8x7 mm. °
. Outline profile of the same. ;

. Similar view of a specimen measuring 21 x 20 mm,

. Outline profile of the same.

. Similar view of a full-grown specimen measuring 24 x 22 mm,
. Outline profile of the same.

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Fig. 7
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Fig. 8a.

PLATE VII
WHITFIELDIA MARIA, Hall
Page 73 rs

- An embryo measuring .75X.75 mm.; without deltidial plates.

Outline profile of the same.

. A later stage of growth, at which the shell measures 5X5 mm., and the deltidial plates have de-

veloped sufficiently to give the foramen a circular outline.

. Outline profile of the same.
. Dorsal view of a normal, mature shell.
. Profile of the same. (28th Ann. Rept. N. Y. State Mus. Nat. Hist., pl. 25, figs. 9, 12.)

MERISTELLA RECTIROSTRA, Hall
Page 67

. The cardinal features at an extremely early stage of growth, the shell measuring 3X2 mm.; show-

ing the broad, triangular foraminal opening and its faintly thickened margins.

. Outline profile of the same. e
. The same features when the shell has attained a size of 4.53.5 mm.; showing the gradual

approximation of the sides of foramen and the narrowing of the umbo, without the formation
_ of deltidial

odikedoattihakal dt TO VOITANVAIIZG

. The same features at maturity.

Outline profile of the same.

Dorsal view of a young individual.

12a, 124. Dorsal, profile, and ventral views of an adult specimen. (11th Ann, Rept. Geol. Surv. -
Ind., pl. 27, figs. 13, 10-12.)

MERISTINA NITIDA, Hall

Page 70
Dorsal view of the youngest individual observed, having a size of 2.51.75 mm.
Outline profile of the same.

. Cardinal features of a shell having dimensions of 3X2 mm.
. Outline profile of the same.
- The same features in an individual having a size of 6X4 mm. In this example the umbo is

abnormally elongate, giving an unusual prominence to the deltidial plates.

. Outline profile of the same. ;
. The same features at a size of 5X4 mm.; showing the total concealment, from incurvature, of the

deltidial plates.
Outline profile of the same.

Figs. 9, 9, 94. Dorsal, ventral and profile views of the normal adult. (28th Ann. Rept. N. Y. State Mus.

Nat. Hist., pl. 25, figs. 3, 4, 5.) ,

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