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Volume 29 CHICAGO, DECEMBER 31, 1943 No. 3

THE AUDITORY REGION OF THE
ARCTOID CARNIVORES

BY WALTER SEGALL

RUSH MEDICAL COLLEGE

In conjunction with a detailed anatomical study of the giant
panda (Ailuropoda melanoleuca) , which is being conducted at Field
Museum, I have had the opportunity of studying the auditory region
of this interesting carnivore. In order to interpret the conditions
found in Ailuropoda it was necessary to examine the auditory region
in other arctoid Carnivora, and the results of that study are pre-
sented here. My studies on the subject were begun in August, 1940,
and continued through 1941 with the support of special funds for
which I am extremely grateful.

Van Kampen (1905), in his monumental treatise on the bony
parts of the auditory region in mammals, combined a wealth of
original observation with a review of the extensive literature up
to that time. He described the auditory region briefly for each of
the families of the Carnivora, but his summary for the order is very
general and devoid of conclusions. Van Kampen's work was never-
theless of great value in the present study, and this was likewise
true of Doran's well-known descriptive catalogue of the auditory
ossicles (1877). Pocock (1921) described the auditory region in the
Mustelidae but approached the problem from the standpoint of
minor rather than major taxonomy. More recently the same author
(1929) gave a very cursory description of the bulla in various pro-
cyonids and bears, but failed to draw any conclusions from his work.
Van der Klaauw (1931) attempted an extensive morphological dis-
cussion of the auditory bulla, based on a review of the literature,
but he arranged his compilation by structural rather than taxonomic
categories in order to supplement van Kampen's work. Hence,
although considerable work has been done on the auditory region in

No. 544 33

NAT.
HlSf.

34 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

this group of animals, it has been scattered and almost wholly
descriptive. No one has attempted to correlate and evaluate, within
the framework of the Arctoidea as a whole, the characters presented.

In most instances variation in the auditory region is not great
within recognized families of the arctoid carnivores. For this reason
descriptions for individual species have been given only in a few
instances. A consistent plan has been followed in describing this
region. First, the lateral bony surface of the auditory region has
been considered, followed by the base of the skull in so far as the
auditory region is a part of it. Then the structures exposed by
a section through the external and internal auditory passages have
been described. The transverse section was slightly oblique in the
Mustelidae, because of the antero-lateral direction of the external
meatus in this family. The ossicles have been described in a separate
section.

Field Museum placed its entire collection of carnivore skulls at
my disposal, and I wish to express my sincerest gratitude for this
co-operation. I am indebted to Mr. Bryan Patterson and Mr. D.
Dwight Davis, both of Field Museum staff, for much assistance and
advice. The drawings on the following pages were prepared with
much care by Miss Peggy Collings, those of the ossicles from camera
lucida outlines made by Miss H. Elizabeth Story.

THE AUDITORY REGION OF THE SKULL

CANIDAE
Dissected skulls examined : Cam's lupus.

Undissected skulls examined: Cam's familiaris, Cam's latrans
(juvenile), Dusicyon thous (bulla opened), Lycaon pictus (bulla
opened).

The Canidae form a more compact group with reference to the
structure of the auditory region than does any other family. The
external meatus is rather short, directed laterad, and oval in cross
section, the longer diameter extending diagonally downward and
forward. Its anterior upper wall is flattened. The outer margin
of the meatus is dentate, and the posterior part of the floor projects
laterally beyond the anterior part. The tympanic forms nearly the
whole circumference of the meatus, only a small gap in its upper
part being completed by the squamosal. In very young individuals
the grooved distal end of the anterior (foliate) process of the malleus
may be distinguished lying in the sulcus malleolaris. The anterior

1943 AUDITORY REGION OF CARNIVORES— SEGALL 35

tip of the ossiculum accessorium mallei lies in the groove of the
foliate process. The ossiculum fuses with the tympanic at a very
early age and is then indistinguishable. The mastoid process is very
small and is formed by the periotic alone. Anteriorly the meatus
borders the posterior face of the glenoid process. The postglenoid
foramen opens on this face of the process, sometimes between the
meatus and the postglenoid process, and sometimes farther dorsad.
The glenoid process is short, its lower end directed only slightly
anteriorly.

The bulla is much inflated in all canids examined.1 Its long axis
is directed from antero-medial to postero-lateral. A diagonal suture,
very distinct in young skulls and often still visible in the adult, fol-
lows the long axis of the bulla, dividing its outer surface into a smaller
medial and a larger lateral part, which, even grossly, differ noticeably
in texture.2 In all species the antero-medial border of the bulla is
nearly parallel to the midline. The antero-medial end of the bulla
partly overlaps the foramen lacerum medium3 and the opening for
the Eustachian tube. The minute Glaserian fissure, which transmits
the N. chorda tympani, is situated on the antero-lateral end of the bulla
laterad of the opening for the Eustachian tube. Posteriorly the
bulla has a sutural union with the prominent paroccipital process,
which is laterally compressed and varies among the different species
in the degree of its posterior extension. Bordering the bulla postero-
medially is the elliptical foramen lacerum posterius, in the anterior
part of which is a foramen that conducts the carotid artery into the
medial wall of the bulla. This foramen is more or less separated by
a low ridge from the inferior petrosal venous sinus lying mesad of it.
Laterad of the posterior end of the bulla, between the posterior wall
of the meatus and the mastoid process, is a pit that contains the
stylomastoid foramen in its anterior lateral part and the recepta-
culum for the tympanohyal in its posterior medial part.

The dorsal surface of the periotic exhibits the usual. pyramidal
shape, and is covered on its lateral side by the tentorium. Medially
an extension of the periotic roofs over the inferior petrosal sinus

1 Except a domestic dog (setter), in which it is rather flat.

2 For a discussion of this condition, see van Kampen (1905, pp. 524-526); and
for a summary of septa in the canid bulla see van der Klaauw (1931, pp. 216-217).

3 This name is used as a convenient topographic term. Current work in the
Field Museum laboratories indicates that various elements of the vascular system
may pass through the foramen lacerum medium. The general conception that it
is intimately connected with the internal carotid artery does not appear to be
true in all cases.

36 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

situated between it and the basi occipital. The internal meatus is
short. There is a hiatus subarcuatus below the superior semi-
circular canal.

The interior of the bulla is encircled by a septum, to varying
degrees in different species, that separates it incompletely into upper
and lower parts; there is always a communication in the middle of
the septum between the two parts of the bulla. A similar septum,
but limited to the anterior part of the bulla, is found in the Muste-
lidae. In the Canidae the septum inserts on the medial wall below
the promontory at the level of the carotid canal, and on the lateral
wall at a lower level than on the medial, so that it is directed diago-
nally downward and laterally. Ridges arising from the ventral
surface of the septum radiate to the middle of the floor of the bulla.

The medial wall of the middle ear contains the canal for the
internal carotid artery, which is situated below and medial to the
promontory. This canal is separated only anteriorly from the canal
for the inferior petrosal venous sinus lying mesad of it; posteriorly
there is no sharp demarcation between them. The very short reces-
sus meatus forms less than a right angle with the short external
meatus.1 The bulla and the alisphenoid participate in the dorsal
part of the anterior wall of the middle ear. The anterior wall is
continued gradually into the tegmen, which is supplied by the
periotic. The latter contains a deep fossa for the M. tensor tympani;
this fossa is prominent in all carnivores. Laterad and slightly
posterior to it is the deep epitympanic recess, which does not con-
tinue into an epitympanic sinus. The floor of the middle ear con-
tinues in a smooth curve into the posterior wall, the upper end of
which turns anteriorly to approach the posterior border of the fenes-
tra rotunda and the lower surface of the promontory, as in the
Ursidae. The entrance of the N. chorda tympani is situated in
the antero-lateral end of the upper edge of the posterior wall. Dor-
sad of the fenestra ovalis is the groove for the facial nerve, which
leaves the middle ear behind the upper part of the posterior wall.

PROCYONIDAE2

Dissected skulls examined: Procyon lotor (figs. 9, 10), Bassariscus
astutus.

Undissected skulls examined: Nasua narica, Bassaricyon medius.

1 Measured by projecting the axis of the meatus internally.

2 Except Polos, which is considered separately (see p. 39).

1943

AUDITORY REGION OF CARNIVORES— SEGALL

37

The external meatus is directed laterally and is usually oval,
with a slightly flattened superior wall. In some individuals there
is an angle between the antero-inferior and postero-inferior walls.

Zygomatic process

Mastoid
process

Paroccipital
process

Bulla

.Postglenoid foramen
»Ext. aud. meatus
.Postglenoid process

Canis nubilis

Procyon lotor

Potos flavus

Tayra barbara

FIG. 9. Lateral views of the auditory region of representative arctoid
carnivores. X 1.

The superior wall of the meatus is formed by the squamosal, the
anterior and posterior walls by the tympanic. The postglenoid
process is directed ventrally and slightly cranially. The anterior
part of the mastoid process is formed by the post-tympanic process

38 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

of the squamosal, the posterior by the pars mastoidea of the periotic.
The mastoid process is very large in Procyon and is separated from
the paroccipital process by a deep notch; it is much smaller in the
other forms. The bulla is well inflated, and its long axis is slightly
curved from antero-medially to postero-laterally. On its surface
there are traces of a suture extending from the opening of the Eusta-
chian tube to the posterior end of the bulla, where the latter touches
the base of the paroccipital process. The bulla gradually slopes up
to the floor of the external meatus. The anterior two-thirds of the
medial border of the bulla (back to the foramen lacerum posterius)
is nearly parallel to the midline, while the caudal third deviates
to the lateral side as in Potos. The opening for the entrance of the
carotid artery varies slightly in position, being sometimes in the
medial bulla wall and sometimes in the suture between the latter
and the basioccipital ; it lies about midway between the spheno-occipi-
tal suture and the elongated foramen lacerum posterius. The stylo-
mastoid foramen is bounded by the bulla medially and anteriorly,
by the mastoid portion of the periotic laterally and posteriorly.
The bulla touches the base of the short, rather pointed paroccipi-
tal process. The foramen lacerum medium, which varies in size,
is situated at the antero-medial end of the bulla. The styliform
process is very short, but projects over the opening of the Eustachian
tube. Between the base of the postglenoid process and the anterior
wall of the external meatus is a postglenoid foramen.

The dorsal surface of the periotic is flattened and pyramid-
shaped; its lateral face is covered by the tentorium. The interns
meatus is very short, and there is no hiatus subarcuatus. The
periotic forms a roof over much of the inferior petrosal sinus situat
on its medial side. The perilymphatic duct opens close to the for
men lacerum posterius.

The inner surface of the bulla is smooth, as in the Ursidae anc
the pandas. It is deepest in the ventro-medial direction, and tl
medial wall curves smoothly into the floor. In the Procyonid?
with the exception of Bassariscus, the bulla exhibits a slightl]
developed oblique septum anteriorly; differences in position occi
within the family. The canal for the carotid artery in the medk
wall is elliptical in section and lies below and medial to the promor
torium. The floor of the bulla ascends anteriorly and is separated b]
a step from the downwardly directed Eustachian tube. The re
of the tympanic cavity is formed by the periotic. A narrow groo^
in the anterior wall leads from the fossa for the tensor tympani

1943 AUDITORY REGION OF CARNIVORES— SEGALL 39

the opening of the Eustachian tube. Laterally and slightly posterior
to this pit is an epitympanic recess, which continues laterally above
the external meatus into an epitympanic sinus. Laterally the bulla
wall comes in contact with the lower surface of the recessus meatus
close to the sulcus tympanicus, but it extends laterally well beyond
the sulcus both anteriorly and posteriorly.

The posterior wall of the bulla is rounded, and its upper end turns
anteriorly to reach the promontorium close to the fenestra rotunda,
as in the Ursidae (not in pandas). The space between the lateral
wall and the promontorium is very narrow. The recessus meatus
and the external meatus are approximately equal in length and
practically in a straight line (Bassariscus) or approaching a right
angle (Procyori). The crista tympanica terminates posteriorly in a
spina tympanica. In the postero-superior wall of the external meatus
is a large fossa which extends latero-posteriorly. It is floored by the
lining of the meatus, and communicates medially with the tympanic
cavity via a minute opening near the malleus.1 Curiously enough,
van Kampen makes no mention of such a suprameatal fossa. The
epitympanic sinus lies above its medial portion. In Bassariscus the
suprameatal fossa is very large and extends into the mastoid process,
which may be completely hollow. A sharp projecting ridge formed
by the upper margin of the tympanic separates it from the posterior
wall. This ridge terminates medially in the spina tympanica pos-
terior. The cavity is exactly comparable to and evidently homolo-
gous with that found in Potos.

The postglenoid foramen is variable; in Bassariscus and Procyon
it is large, and a spicule of bone often extends across it from the
anterior wall of the meatus, dividing the foramen more or less com-
pletely into two.

Dissected skull examined: Potos flavus (figs. 9, 10).

The external auditory meatus is oval in cross section, and directed
laterally. Its entrance is narrowed anteriorly by the thickened end
of the anterior part of the tympanic, posteriorly by the lateral end of
a ridge running transversely along the middle of the posterior wall.
The upper wall is concave ventrally and extends laterad beyond the
lower one. The tympanic supplies the lower half of the meatus, the
squamosal the upper half. The anterior lateral end of the floor is
thickened and turned slightly anteriorly. The postglenoid foramen
is very small. The mastoid process is rudimentary and agrees with

1 The relations of this area were checked on a preserved specimen. F.M.N.H.
No. 1304.

40 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

the other Procyonidae in that it shows no trace of the separate bony
element present in the Ursidae and Mustelidae. The bulla is less
inflated than in any other procyonid, and continues without external

Epityrnpanic sinus

Sinus in
ext oud meat

Sinus i n
exf. and. meat us

Ext.aud. meat us

Promontoriu

Int.aud. meatus
Cochlea

ulla

Procyon lotor

Potos flavus

Entronce

sinu

Sinus

below meatus

Septum

Entrance to mastold sinus

Taxidea taxus

Posterior extension
of tympanic cavity

Tayra barbara

FIG. 10. Transverse sections through auditory region of representative
arctoid carnivores. Posterior view. X 1.

demarcation into the short meatal floor. The medial border of the
bulla is parallel to the midline, only its posterior end deviating
slightly to the lateral side, as in Procyon. The posterior carotid
foramen is situated far anteriorly in the medial wall of the bulla,
close to the spheno-occipital suture, at about the same level as in

1943 AUDITORY REGION OF CARNIVORES— SEGALL 41

the Mustelidae. Consequently the carotid canal is short and its
first part is directed entirely dorsally. Its upper part turns anteriorly
at a right angle to open into the cranial cavity at the large foramen
caroticum internum. This foramen is connected by a narrow canal
with the foramen lacerum medium. The opening of the Eustachian
tube is invariably overlapped by a pointed styliform process. The
foramen lacerum posterius is elongated, with its long diameter from
anterior medial to posterior lateral. The posterior end of the bulla
touches the base of the short paroccipital process, whose apex is
directed ventro-posteriorly. The stylomastoid foramen is bounded
anteriorly and medially by the bulla, laterally and posteriorly by
the pars mastoidea of the periotic, as in the Procyonidae, Ursidae,
Ailuropoda, and Ailurus. The external face of the latter is in a
nearly horizontal plane. The lateral caudal part of the bulla slopes
down to this part of the periotic. In the other Procyonidae the bulla
is well inflated, and the pars mastoidea of the periotic is in a vertical
plane.

The dorsal face of the periotic exhibits a hiatus subarcuatus. The
internal meatus is very short. The sulcus on the medial border of
the periotic is roofed over by the basioccipital to form a canal for the
inferior petrosal venous sinus. The foramen caroticum internum is
wide and oval in cross section.

The bulla is slightly compressed transversely, and internally
shows a few low ridges running from the annulus tympanicus to the
medial wall as in the Mustelidae. The anterior ascending part of
the floor continues over a low step into the floor of the Eustachian
tube. Posteriorly the bulla extends well behind the promonto-
rium to the base of the paroccipital process, as in the Mustelidae.
The promontorium is more pronounced than in Procyon. The reces-
sus meatus is almost in a straight line with the extremely short
external meatus. The crista tympanica ends posteriorly in a promi-
nent spine, which continues laterally into the ridge projecting along
the posterior wall of the meatus. Above this ridge the superior part
of the posterior wall and the bordering part of the superior wall bound
a suprameatal fossa which is shallower than in other Procyonidae.
This cavity is immediately lateral to the recess. Posteriorly and
medially the spina tympanica posterior continues into a "Chorda-
fortsatz" that projects in front of the foramen for the entrance of the
N. chorda tympani into the middle ear. The spina tympanica anterior
is less developed than the posterior. The upper wall of the middle
ear contains an epitympanic recess. There is no epitympanic sinus.

42 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

AILURIDAE

Dissected and undissected skulls examined: Ailurus fulgens,
Ailuropoda melanoleuca.

Owing to the difference between these two forms, it has been
found more convenient to describe them separately.

Dissected skull examined : Ailurus fulgens.

The external meatus is directed laterally and is oval in cross
section, with the lower half formed by the tympanic and the upper
by the squamosal. The apex of the postglenoid process is directed
anteriorly as in Ailuropoda. The postglenoid foramen is large and
directed ventrally. As in Ailuropoda, the canal leading from this
foramen to the transverse sinus is very well developed. The mastoid
process is small and situated behind the external meatus. Its
anterior part is supplied by the squamosal, its posterior part by the
pars mastoidea of the periotic, which forms the apex of the process.
The bulla is moderately inflated; its medial border is straight and
runs slightly postero-laterally. The posterior carotid foramen is in
front of and very close to the foramen lacerum posterius. The latter
is separated from the stylomastoid foramen by the pointed posterior
end of the bulla, which touches the base of the paroccipital process.
The paroccipital process is relatively long and directed ventrally and
posteriorly, as in Ailuropoda. The bulla projects anteriorly with a
well-developed styliform process over the opening for the Eustachian
tube. Mesad of this opening is the large foramen lacerum medium,
which leads through a short, posteriorly directed canal into the
skull.

A hiatus subarcuatus is present on the dorsal surface of the peri-
otic. The carotid canal is enclosed in the medial wall of the tympanic
cavity, medial to and below the promontory. The medial wall forms
an angle with the floor. The recessus meatus forms less than a right
angle with the external meatus. The crista tympanica ends anteri-
orly and posteriorly in a small spine. Anteriorly the floor of the
tympanic cavity rises a little to continue over a step into the Eusta-
chian tube. The floor continues in a gentle curve into the vertical
posterior wall, and this is separated by a fissure from the periotic,
which forms the upper part of the posterior wall, as in Ailuropoda.
The entire tegmen is formed by the periotic; in the Procyonidae and
Ursidae the posterior wall of the tympanic turns anteriorly to form
a small part of the tegmen. There is no epitympanic sinus, in con-
trast to the Procyonidae.

Four skulls (one sectioned) examined: Ailuropoda melanoleuca.

1943 AUDITORY REGION OF CARNIVORES— SEGALL 43

The external auditory meatus is directed laterally and nearly
horizontally. The tympanic forms its floor and the lowest part of
the anterior and posterior walls; the remainder and by far the greater
part is formed by the squamosal. The postglenoid process has
roughly the shape of a triangular pyramid, whose apex is directed
anteriorly. The postglenoid foramen leads into a canal that opens
into the groove for the transverse sinus. The mastoid process is well
developed, its anterior part formed by the squamosal, its posterior
part by the mastoid process of the periotic. It is directed ventrally
and slightly anteriorly. It is separated from the paroccipital process
by a wide notch in which runs the suture between the mastoid process
and the exoccipital. When seen in section the mastoid contains no
cells and is spongy in texture. The short paroccipital process con-
tinues onto the basicranium as a ridge running anteriorly and medi-
ally. The enlarged anterior end of this ridge has a sutural union with
the tympanic, and the foramen lacerum posterius is situated immedi-
ately medial to this suture. Anterior to the paroccipital process is a
large pit, the vagina processus hyoidei, bounded medially and
posteriorly by the exoccipital, laterally by the mastoid process, and
anteriorly to a slight extent by the tympanic. Immediately antero-
laterad of the vagina is the facial canal; the wall between the two
at this point is very thin and incomplete, and is formed in part by
the periotic and in part by the tympanic.

The ventral surface of the auditory region is formed by the
extremely flat tympanic. This bone is not inflated and there is no
bulla in the literal sense. It is directed from antero-medial to
postero-lateral. Antero-medially the tympanic forms part of the
Eustachian tube, of which it provides the floor, while the alisphe-
noid supplies the roof. There is a well-developed styliform process,
which covers the Eustachian tube ventrally. The foramen lacerum
medium is variable; it may be either present or absent, or present
on one side of the skull only. Lateral to the tube and near the antero-
medial angle of the base of the postglenoid process is the opening of
the canal for the N. chorda tympani.

The dorsal surface of the periotic is roughly pyramidal in shape.
The greater part of its lateral surface is covered by the alisphenoid,
and its uppermost part is in contact with the tentorium cerebelli
formed by the parietal. Behind and slightly below the orifice of the
transversely directed internal meatus, and close to the anterior
border of the foramen lacerum posterius, is the slit-like opening for
the perilymphatic duct. The carotid canal begins inside the foramen

44 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

lacerum posterius, runs forward anteriorly and slightly medially
inside the medial wall of the tympanic cavity, and opens into the
cranial cavity through the foramen caroticum internum. This open-
ing is lateral to the pointed anterior end of the periotic. Lateral to
it is the opening of a small canal that is roofed by the periotic and
floored by the alisphenoid. In transverse section the tympanic
cavity has the shape of a rhomboid whose long axis is directed
ventro-medially-dorso-laterally, forming an angle of about 45° with
the horizontal. The floor is flat except for a very low, antero-pos-
teriorly directed ridge. The angle between the floor and the posterior
wall is rounded. The lower part of the posterior wall is formed by
the tympanic, the upper part by the periotic, separated from each
other by a fissure as in Ailurus. There is no demarcation between
the tympanic cavity and the low hypotympanic sinus. The latter
extends laterally for a very short distance under and on both sides
of the sulcus tympanicus, so that the sulcus projects into the
tympanic cavity. The floor of the cavity continues evenly into the
floor of the Eustachian tube, which is divided by a low, sharp, bony
ridge into a smaller lateral and a larger medial part.

The lower part of the medial wall of the tympanic cavity is formed
by the tympanic, which contains the carotid canal in its dorsal part.
The upper part of the medial wall is formed by the petrous portion
of the periotic. The fenestra ovalis is separated from the promon-
torium by a small groove that continues posteriorly for about 3 mm.
into a spout-like process, projecting above the entrance of the N.
chorda tympani. Close to the middle of the anterior border of the
promontorium is the beginning of a small canal that opens into the
cranial cavity lateral to the internal carotid foramen as described
above. The lateral wall of the tympanic cavity is formed by the
recessus meatus with the projecting sulcus tympanicus. There is a
slightly developed spina tympanica posterior. The "Chordafortsatz"
is not marked, but there is a small bony ridge that conducts the N.
chorda tympani anteriorly from its entrance into the tympanic
cavity. The recessus meatus forms a little less than a right angle
with the external meatus. Anterior to the anterior upper end of the
crista tympanica is the orifice of the canal through which the N.
chorda tympani leaves the middle ear. The lateral wall of the
epitympanic recess and the tegmen tympani are supplied by the
squamosal, the medial wall by the periotic. There is no epitympanic
sinus. The epitympanic recess is separated from the posterior por-
tion of the middle ear by a small bony septum, which extends nearly

1943 AUDITORY REGION OF CARNIVORES— SEGALL 45

transversely. Posterior to it is a poorly developed cavity, through
which the facial nerve passes toward its exit from the middle ear.

URSIDAE
Dissected skulls examined: Euarctos americanus.

Undissected skulls examined : Ursus arctos, Thalarctos maritimus,
Tremarctos ornatus, Melursus ursinus (juvenile).

The external meatus is long and directed laterally and slightly
anteriorly. It is oval in cross section, with the greater diameter
dorso- ventral. The external part of the canal projects freely. The
tympanic forms the floor, the anterior wall, and a small part of
the posterior wall of the meatus. The roof and the greater part of the
posterior wall are supplied by the squamosal. The mastoid process
is formed anteriorly by the squamosal, posteriorly by the mastoid
process of the periotic; a third, wedge-shaped bone is interposed
ventrally between these two. This element extends farther dorsally
on the external side than on the internal. The mastoid process has
a spongy structure. The postglenoid process is directed antero-
ventrally to a slight extent.

The ventral surface of the auditory region is extensive and has
the shape of a triangle, the apex of which is formed by the external
meatus and the base by the tympano-basioccipital suture. This
suture is nearly parallel to the median line of the skull. Only a small
anterior part of the median border of the bulla is in contact with the
basisphenoid. Externally the bulla is flattened, which is effected by
thickening of the floor of the external meatus. The paroccipital
process does not touch the bulla, but is connected with it by a short
ridge that runs postero-laterally. The foramen lacerum posterius is
oval and is sometimes partially divided into anterior and posterior
parts. The posterior orifice of the carotid canal is ventral and
slightly anterior to the foramen lacerum posterius and is visible
externally in all forms examined. Immediately dorsal to the pos-
terior carotid foramen is the opening of a small canal, which leads
into the middle ear to open beneath the promontorium. Lateral to
the foramen lacerum posterius, and separated from it by the ridge
connecting the bulla and the paroccipital process, is a large pit
bounded by the tympanic medially and anteriorly, the mastoid proc-
ess laterally, and the paroccipital process posteriorly. Its postero-
medial part houses the tympanohyal, and the stylomastoid foramen
opens into its deep antero-lateral part. The carotid canal opens at
the antero-medial angle of the bulla into a large orifice which it shares

46 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

with the foramen lacerum medium. From this orifice the short canal
of the foramen lacerum medium, for the passage of the internal caro-
tid artery, leads dorsally and a little posteriorly into the cranial
cavity. The foramen lacerum medium is plainly visible externally in
some skulls, but is almost hidden in others due to the antero-medial
growth of the bulla. This variability is considerable, even among
specimens of the same species. The internal orifice of the canal (the
foramen caroticum internum) is large and is situated medial to the
apex of the periotic pyramid. The opening of the Eustachian tube
is lateral to the foramen lacerum medium; it is covered by an acute
styliform process. The opening in the fissure Glaseri for the passage
of the N. chorda tympani is between the bulla and the postero-medial
corner of the postglenoid process. The postglenoid canal opens
between the posterior side of the base of the postglenoid process and
the tympanic bone. The axis of the canal is directed ventrally and
slightly cranially.

The dorsal surface of the periotic is slightly concave. The inner
meatus is short. The septum which divides it into upper and lower
parts is plainly visible from the outside. Under the superior semi-
circular canal there is a large fossa subarcuata. Behind and slightly
medial to the inner meatus is the slit-like opening of the perilympha-
tic duct, which continues into a groove leading to the foramen lac-
erum posterius.

Despite the external flatness of the bulla, the hypotympanic
sinus (i.e. the cavity of the bulla) is quite well developed. Its internal
surface is smooth, and its medial wall continues into the floor in a
smooth curve. The carotid canal is enclosed in the medial wall of
the bulla and is situated medial to and slightly below the promon-
torium. Anteriorly, the medial wall of the hypotympanic sinus
approaches the lateral wall, so that the floor of the tympanic is
narrow there. It continues over a slight bony ridge into the steeply
antero-ventro-medially directed floor of the Eustachian tube. Pos-
teriorly the floor continues in a smooth curve into the posterior wall.
The upper part of the latter turns anteriorly to meet the periotic
dorsad of the fenestra rotunda. Between them a small opening is
left, through which the facial nerve leaves the middle ear. The hori-
zontal semicircular canal is above and slightly lateral to this cavity.
In the upper end of the posterior wall is the opening for the entrance
of the N. chorda tympani into the tympanic cavity. The periotic
encloses the epitympanic recess and forms the tegmen tympani.
The lateral wall of the recess is formed by the squamosal, which is

1943 AUDITORY REGION OF CARNIVORES— SEGALL 47

cancellous medially. There is no epitympanic sinus. Anteriorly and
slightly medially to the epitympanic recess is a triangular fossa for
M. tensor tympani whose apex is directed anteriorly and continues
in a ridge running along the anterior wall of the tympanic as far as
the ostium tympanicum of the Eustachian tube. The recessus
meatus is short, and its plane forms almost a right angle with the
axis of the external meatus. The upper anterior end of the crista
tympanica ends in a slight spine. The recessus meatus and the
external meatus proper form a blunt angle with each other.

MUSTELIDAE

Dissected skulls examined: Tayra barbara (figs. 9, 10), Lutra cana-
densis, Taxidea taxus, Helictis taxilla, Conepatus mesoleucus.

Undissected skulls examined: Mustela nigripes, Gulo luscus,
Mephitis mesomelas, Meles meles, Arctonyx collaris.

The external auditory meatus is directed laterally and also has a
decided anterior inclination. It is either round or oval in section,
with the greater diameter in the antero-posterior direction. The
extent to which the tympanic participates in the formation of the
meatus is variable, but it appears to form more than the lower half
in all cases. The posterior face of the postglenoid process is greatly
elongated in all forms studied, except the Melinae. There is great
variation in the degree of inflation of the bulla. It may be almost
flat, as in Arctonyx, or greatly inflated, as in Taxidea. The external
surface is frequently marked by shallow transverse grooves, between
which a slight bulging is apparent. The anterior part of the mastoid
process is formed by the squamosal, the posterior part by the periotic.
Between them, at the apex, is a small ossification center, comparable
to that found in the Ursidae, which usually co-ossifies completely
with the other two components. The bulla wall continues without
any sharp demarcation into the external wall of the meatus. The
medial border of the bulla deviates postero-laterally to a slight
extent. The posterior carotid foramen opens near the center of the
medial wall at the level of the spheno-occipital suture, slightly farther
caudad than in Potos; there is some variation in the position of this
foramen, although in general it is situated in the anterior half.
Behind it in some forms (Lutra, Tayra) is a foramen that leads
through a short canal into the groove for the inferior petrosal venous
sinus on the dorso-medial side of the periotic. The styliform process
may be entirely wanting, as in Taxidea, or greatly developed, as in
Mephitis. The position of the postglenoid foramen varies in correla-

48 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

tion with differences in the shape and size of the bulla and postglenoid
process. The foramen lacerum posterius, in comparison with that of
other arctoids, is slightly narrowed antero-laterally by the projecting
bulla. The stylomastoid foramen is bounded by the mastoid process
laterally and by the tympanic on the other three sides. This is
apparent in young skulls, but in adults the foramen is enclosed by the
bulla to such a degree that it appears to be completely surrounded
by the tympanic. This condition is characteristic of the family. In
the majority of the forms there is a foramen anterior to the stylo-
mastoid foramen, sometimes enclosed with it in a common pit, that
leads into the floor of the external meatus. There is sometimes a slit
in the floor of the meatus leading to this foramen, which may partly
separate the posterior wall of the meatus from the floor. The paroc-
cipital process varies in its development. Its extremity may be either
in close contact with the posterior end of the bulla (Tayra) or well
separated from it (Mephitis). Sometimes there is a vascular canal
between the process and the bulla. A hiatus subarcuatus is present
in the cranial surface of the periotic. Between the paroccipital proc-
ess and the stylomastoid foramen, the bulla and the mastoid are
fused. This fusion does not take place in other arctoids, including
Potos.

The cavity of the middle ear continues posteriorly well beyond
the promontorium in members of this family. In this respect the
mustelids differ in degree from other arctoids except Potos. In addi-
tion, as Pocock (1921) pointed out, in some cases the mastoid process
contains a sinus that is in communication with this posterior part of
the tympanic cavity; this sinus reaches an extreme in Taxidea, in
which the mastoid is visibly inflated. In other genera this cavity is
lacking; in some forms in which the bulla is considerably inflated (e.g.
Helictis), the posterior portion of the tympanic cavity, which is
bounded anteriorly by a transverse septum in the vicinity of the pro-
montorium (Pocock's "posterior rafter"), superficially resembles a
mastoid sinus. The latter condition is characteristic of the Muste-
linae and Lutrinae, while the sinus is present in the Mephitinae and
in the Melinae with the exception of Meles. Pocock (I.e., p. 486)
regards conditions in Meles as representing more or less the ancestral
structure from which that of the skunks and badgers may be derived.
Descriptions and an adequate summary are given by this author.

In section the bulla exhibits various degrees of inflation. In all
species examined, ridges (Pocock's "rafters") of varying height and
number arise from the under surface of the recessus meatus and

1943 AUDITORY REGION OF CARNIVORES— SEGALL 49

radiate to the medial wall, dividing the lower part of the bulla into
several compartments that are open dorsally. An oblique septum
arises from the anterior wall of the tympanic cavity, broadens later-
ally, and reaches to the anterior lower margin of the recessus meatus.
This structure appears to be characteristic of the Mustelidae but is
quite variable even between individuals of the same species. It
is similar to the septum found in the Canidae. In some forms it is
very small (Lutra), in others large (Taxidea). It divides the most
anterior part of the middle ear into ventral and dorsal parts. The
tympanic orifice of the Eustachian tube is situated in the dorsal
part. The anterior wall of the middle ear is partly supplied by the
alisphenoid, which here forms a nearly transverse suture with the
periotic. The promontorium is attenuated posteriorly, differing
slightly in this respect from the other arctoids.

The epitympanic recess continues posteriorly and laterally from
the upper and dorsal part of the posterior wall of the external meatus
into a sinus. This sinus is laterally directed, completely separated
from the posterior extension of the tympanic cavity, and varies in
size and extent in the different forms. The recessus meatus is con-
tinuous with the floor of the external auditory meatus in some forms
(e.g. Lutra and Conepatus), while in others it forms less than a right
angle (Taxidea). Between the floor and the posterior wall of the
meatus, near the border of the recessus and the meatus proper, is a
foramen that opens on the external side close to the stylomastoid
foramen. In Taxidea there is another air space, triangular in sec-
tion, below the whole length of the external meatus. It is closed
anteriorly, but posteriorly communicates through a wide opening
with the posterior extension of the tympanic cavity.

DISCUSSION OF AUDITORY REGION

Comparison of the structure of the auditory region in the arctoid
Carnivora reveals only six characters that are sufficiently funda-
mental and consistent within a given family to be compared through-
out the group. These are:

(1) The external auditory meatus, its angle of inclination, and
the elements of which it is composed.

(2) The bones participating in the formation of the stylomastoid
foramen.

(3) The presence or absence, and the position when present, of a
septum or septa in the bulla (not, however, a septum bullae).

50 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

(4) The presence or absence of a sinus in the vicinity of the
external auditory meatus, and its position when present.

(5) The position of the posterior carotid foramen.

(6) A marked extension of the tympanic cavity posterior to the
promontorium (characteristic of the Mustelidae and Potos only; see
footnote, p. 51).

Other characters of the auditory region, such as the shape and
degree of inflation of the bulla, the relations of the recessus meatus,
the positions of the vascular foramina (except the posterior carotid
foramen), and the various processes, appear to be too variable within
families to be of taxonomic importance.

On the basis of the six characters enumerated above it is possible
to divide the arctoid carnivores into three main groups. These are
(1) the Canidae; (2) the Procyonidae, Ailuridae, and Ursidae; and
(3) the Mustelidae. The auditory structure of Potos is intermediate
between groups 2 and 3, and is discussed separately below. These
groups are characterized as follows:

(1) Canidae.— External auditory meatus directed laterally,
formed almost entirely by the tympanic (in young skulls the squa-
mosal participates to a very slight extent dorsally); stylomastoid
foramen bounded by the bulla anteriorly and medially, by the mas-
toid laterally and posteriorly; an obliquely directed horizontal
septum in the anterior part of the bulla, sometimes running com-
pletely around the cavity with an aperture in the center (ridges may
radiate from this septum to the middle of the floor) ; no sinus in the
vicinity of the meatus; posterior carotid foramen lying in the vesti-
bule of the foramen lacerum posterius; no post- tympanic sinus.

The Canidae form an extremely compact group, apparently
containing no aberrant forms. They are sharply set off from group 2
in three of the six characters under consideration; two of the
remaining three characters differentiate the Canidae, Procyonidae,
Ailuridae, and Ursidae from the Mustelidae.

The fact that the Canidae form such a distinct and compact
group seems to demand reconsideration of the view that the Ursidae
have been derived from this family. Comparison of the external
auditory region of the living Canidae with that of the Oligocene
Pseudocynodictis reveals very great similarity in the shape and
direction of the external auditory meatus, in the bulla, and in the
glenoid and paroccipital processes.

(2) Procyonidae, Ailuridae, and Ursidae. — External auditory
meatus directed laterally except in the Ursidae in which it has a

1943 AUDITORY REGION OF CARNIVORES— SEGALL 51

slight antero-lateral inclination, formed in large part by the squa-
mosal, which contributes the roof and the dorsal portions of the
sides; stylomastoid foramen as in the Canidae; no septum in the bulla,
except to a very slight degree in the Procyonidae with the excep-
tion of Bassariscus (see p. 38); no sinus in the vicinity of the
meatus except in the Procyonidae, where it is situated in the postero-
superior wall of the external meatus; posterior carotid foramen
either as in the Canidae (Ailuridae and Ursidae) or in the posterior
half of the medial border of the bulla (Procyonidae); no post-
tympanic sinus.

The members of this group are more diverse than the Canidae,
as might be expected; however, the families included resemble each
other more closely than they do any member of the other two
groups.

(3) Mustelidae. — External auditory meatus directed antero-
laterally, participation of tympanic variable but always forming
more than the ventral half; stylomastoid foramen almost completely
surrounded by the tympanic (in the adult skulls examined), and
hence appearing as though situated in the midst of the bulla; an
obliquely horizontal septum in the anterior part of the bulla, always
accompanied by one or more approximately transverse ridges; a
sinus, variously developed but always present, situated behind the
upper part of the posterior wall of the external meatus; posterior
carotid foramen almost always in the anterior half of the medial
border of the bulla; tympanic cavity continued posteriorly well
beyond promontorium;1 a sinus in the mastoid occurs in Mephitinae
(except Meles) which communicates with the posterior portion of the
tympanic cavity; bulla and mastoid fused between paroccipital
process and stylomastoid foramen.

The Mustelidae are rather compact in the diagnostic characters,
but in the other features of the auditory region they are extremely
diverse.

Polos has been omitted from the above discussion because of the
peculiar mixture of characters presented by its auditory region. This
genus resembles the members of group 2 (and hence differs from the
Mustelidae) in the composition and direction of the external meatus
and in the bones taking part in the formation of the stylomastoid
foramen. It agrees with the Procyonidae proper in the position of

1 This is a difference of degree from other arctoids. The canids, and Nasua and
Bassaricyon among the procyonids, show a posterior extension of the cavity but
not to so marked an extent as in the mustelids.

52 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

the sinus in the posterior and superior walls of the external meatus,
and in the projecting ridge that separates this space from the lower
part of the posterior wall. The relations of this cavity are of con-
siderable interest. Although in Potos and the Procyonidae proper
it occupies a different position from the corresponding cavity in the
Mustelidae, there can be little doubt that this structure is homolo-
gous in the two groups, when the differences introduced by the
different direction of the meatus are discounted. The fact that
Potos, and to a lesser extent the Procyonidae proper, tend to connect
group 2 with the Mustelidae may be significant phylogenetically.1

Potos shares the following characters with the Mustelidae: the
anterior position of the posterior carotid foramen, the presence of
approximately transverse ridges on the bulla floor, and the pro-
nounced extension of the tympanic cavity well beyond the promon-
torium. Other mustelid characters found in the ossicles are discussed
on page 54.

Obviously, determination of the exact relations of Potos is
extremely difficult. In the auditory region resemblances to the
Mustelidae are important and are perhaps more significant than are
the characters it shares with the Procyonidae or with group 2 in
general. It should be mentioned that Bassaricyon differs from Potos
in all those characters of the auditory region in which the Procy-
onidae proper differ from it. Pending the results of detailed anatomi-
cal studies, one can only say that on the basis of present evidence
Potos may perhaps be regarded as a marginal member of the muste-
loid stock that has retained certain characters from the common
arctoid ancestry.

THE OSSICLES

Doran stated that "in the Fissipedia the distinctive features of
any clear value in classification are chiefly confined to the malleus."
My own observations are in entire agreement with this statement,
and accordingly the descriptions given below are restricted chiefly
to this ossicle. As the work progressed it became apparent that the
dorsal contour of the head of the malleus was of the most taxonomic
significance, and this aspect is therefore emphasized both in the
descriptions and in the illustrations. Other features of the ossicle
are variable, but the dorsal contour is remarkably constant among
the members of a family. Aberrations in this aspect in any member

1 It may be mentioned in this connection that the skull of Zodiolestes, from
the lower Miocene of Nebraska, combines a mustelid dentition with a procyonid
auditory region (Riggs, 1942).

Lamina

Manubrium

Vertical

ridge

Lateral
surface

Muscular
process

Manubrium

facet)

Lower facet] plane
Neck

Manubrium

Anterior Posterior

Canis familiaris

Procyon lotor

Ursus horribilis

Potos flavus

Ailuropoda melanoleuca

Grison cuja
Lutra canadensis

FIG. 11. Mallei of representative arctoid carnivores. Various scales.

53

54 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

•of a group are accompanied by differences in the general auditory
structure (e.g. Potos and the Melinae).

Canidae. — The mallei of Cam's familiaris, C. nubilis, C. latrans,
Dusicyon culpaeus, and Lycaon pictus were examined. The malleus
has a very characteristic appearance among these sp'ecies. The head
seen from above is triangular, with the base directed laterally, the
apex medially. There is no prominence in the region of the lamina.
The upper and lower facets of the articulation plane form a little
more than a right angle with each other. The vertical ridges dividing
the facets into outer and inner parts are not well developed. The
lower facet is rather oval, in contrast with the triangular form in the
Procyonidae. The neck is short and slightly bent; the base from
which it arises is broad. The processus gracilis is short and nearly
straight. The lamina is correspondingly small and inserts above
at the lower circumference of the head. The muscular process is
well developed and is directed medially, ventrally, and anteriorly.
The manubrium is triangular in cross section and is bent anteriorly.
The lateral face of the manubrium is uniform in width throughout
its whole length, a condition unique among the carnivores examined.
The anterior and posterior faces have the shape of a long narrow
triangle with the base above and the apex below.

Procyonidae. — The mallei of Bassariscus astutus, Nasua narica,
and Procyon lotor were examined. They closely resemble the mallei
of the Ursidae, and also show great similarity to those of Ailuropoda
and Ailurus. Seen from above the head is roughly pear-shaped,
particularly in Procyon and Bassariscus, with a round prominence
postero-medially. Another prominence on the anterior side of the
head serves for the insertion of the upper end of the lamina. The
upper and lower facets of the articulation plane form an obtuse
angle. The lower facet is triangular, with the apex directed medio-
ventrally. The vertical ridge is only slightly developed in Procyon,
but more so in Bassariscus and still more in Nasua. The neck of the
malleus is straight in Procyon, but slightly bent in Nasua and Bas-
sariscus. The lamina is well developed. The muscular process is
very short, particularly in Bassariscus. The manubrium is straight,
except that its lower end is spatulate and slightly bent laterally.

Potos flavus. — The head of the malleus is globular in shape, in
contrast to that of the Procyonidae proper. It recalls the contour
of the head in the mustelids Taxidea and Mellivora. Postero-
medially it has a ventrally directed prominence. The upper and
lower articulation facets form a blunt angle. The lower one is tri-

Canis familiaris

Medial

Canis nubilis

Lycaon pictus

Bassariscus astutus Procyon lotor Nasua narica Potos flavus

Ailuropoda melanoleuca

Melursus ursinus

\J

Mustela vison Martes pennanti Gulo liiscus Grison cuja

s-^^ m/

^jtoji^jjP

Lutra canadensis Arctonyx collaris Taxidea taxus

FIG. 12. Dorsal views of mallei of representative arctoid carnivores. Various
scales.

56 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

angular, as in the Procyonidae. The vertical ridge is relatively well
developed. The lamina is small, and the muscular process is slightly
larger than in the Procyonidae. The manubrium is bent. Its ante-
rior and posterior faces are broader than in the Procyonidae. Its
lateral face is very narrow but expands slightly below.

Ailurus fulgens. — The malleus is similar to that of Ailuropoda.
The head is very small laterally and enlarges medially. Antero-
medially it projects for the attachment of the upper part of the
lamina. The upper and lower facets form a blunt angle, particularly
in their outer parts. The lower facet is rather extensive. The verti-
cal ridge is more developed in the lower facet than in the upper. The
neck is curved in the antero-posterior and latero-medial planes, and
hence the lamina is short and narrow. The muscular process is very
short. The manubrium of the malleus is straight except that its
lower end is bent slightly laterally. Its anterior and posterior faces
have the shape of a small triangle with the base above, as in Procyon.
Its lateral face is very narrow, broadening below to give the end a
spatulate appearance.

Ailuropoda melanoleuca. — The malleus is similar to that of
Ailurus. The head seen from above is pear-shaped, but its lateral
face is broader than in Ailurus. There is a slight prominence at the
anterior side of the head where the lamina inserts. The upper and
lower facets of the articulation plane form an obtuse angle with each
other. The vertical ridge is prominent, particularly in the lower
facet. The latter is triangular and is divided by the vertical ridge
into medial and lateral parts, which form a right angle with each
other. The neck is slightly curved in a latero-medial direction, and
the lamina arising from it is well developed. The base is broad.
The muscular process is hardly visible. The manubrium has broad
anterior and posterior faces, which become very attenuate below.
Its lower end is rather sharply bent laterally and expands slightly.

Ursidae. — The mallei of Thalarctos maritimus, Melursus ursinus,
and Euarctos americanus were examined. They are very similar.
The head of the malleus is flat on the posterior side, rounded on the
anterior side. The upper and lower articulation facets form an angle
of more than 90° with each other. The vertical ridge is particularly
well developed on the lower articulation facet. The neck is nearly
straight and forms only a slight angle with the head. The lamina is
well developed. The base of the manubrium is broad. The muscular
process is very small or absent, and there is no processus brevis.
The manubrium is straight, with broad anterior and posterior planes.

1943 AUDITORY REGION OF CARNIVORES— SEGALL 57

Distally the extremely narrow lateral plane expands into a spatulate
tip, which is bent slightly laterally.

The incus is characterized by the shortness of the processus
brevis, as in the Mustelidae.

Mustelidae. — The mallei of Mustela, Martes, Grison, Lutra,
Mephitis, Helictis, Arctonyx, Mellivora, and Taxidea have been
examined. They show, except in the shape of the head, more differ-
ences among each other than does any other group of the arctoids.
The contour of the head is very characteristic. It is as a rule com-
pressed in its antero-posterior direction, only slightly expanded
medially. The heads of Taxidea and Mellivora differ in that they
are rather broad and globular. The convex part of the head is
extensive in some forms (e.g. Grison) . In the latter the medial lower
end turns dorsally again like a letter J. The long diameter of the
articulation plane is greater in the Mustelidae than in any other
group of carnivores. It shows great differences among the species
examined in its upper and lower facets with reference to the shape
of the facets and development of the vertical ridge. The lower facet
is tongue-shaped in most forms, sometimes more elongate (Lutra)
sometimes more oval (Mellivora). The vertical ridge is generally
little developed (Taxidea), in some forms hardly visible (Lutra).
Very often it is present only in the lower facet. The neck varies
considerably in length ; sometimes it is short (Lutra), occasionally long
(Taxidea). The lamina is short and narrow. There is always a well-
developed muscular process. The processus brevis if present at all
is very short. The manubrium has broad anterior and posterior
planes. Sometimes it is straight, sometimes slightly bent; the lower
end is spatulate. In Meles there is a conical prominence on the neck
opposite the origin of the processus gracilis, which was described by
Wilkie (1926), who also discussed the broader morphological impli-
cations of this structure. I find this prominence in Mellivora as well
as in Meles, but can detect no trace of it in Taxidea.

DISCUSSION OF OSSICLES

On the evidence of the ossicles the arctoid carnivores appear to
fall into the same three groups as were indicated by the general
auditory structure. Here also Potos is intermediate.

(1) Canidae. — Contour of head triangular with base lateral and
apex medial, no prominence in region of lamina; lamina rather long;
long diameter of articular surface relatively short, lower facet more or

58 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. 29

less oval; muscular process well developed; manubrium bent anteri-
orly, with uniformly broad lateral face.

(2) Procyonidae, Ursidae, and Ailuridae. — Contour of head pear-
shaped or irregularly quadrilateral, always broader medially than
laterally; slight prominence anteriorly, varying from central to
medial in position, at or near to insertion of lamina; lamina generally
long, as in group 1, but anterior end more pointed; long diameter of
articular surface about as in Canidae, lower facet varying from tri-
angular (Procyonidae) to oval (Ursidae, Ailurus, Ailuropoda) ; mus-
cular process very small; manubrium only slightly bent, spatulate
distally.

(3) Mustelidae. — Contour of head a flattened oval with nearly
parallel sides (except in the Melinae in which the anterior side is
rounded). No prominence in region of lamina; lamina short and
narrow, approaching head more laterally than in members of group
2; articular surface extremely variable in shape and extension, but
long diameter generally greater than in groups 1 and 2; muscular
process well developed, as in group 1; manubrium in most cases
slightly bent, occasionally straight, spatulate distally.

The first group, the Canidae, is very compact, all the members
agreeing closely in every character. The second group is rather more
diverse. The Ursidae and the Procyonidae proper are very similar.
Potos exhibits some differences from both these families in the dorsal
contour of the head of the malleus. Ailurus and Ailuropoda exhibit
certain slight differences from each other and from the other genera
of this group, but are by no means as distinct as is Potos. The third
group is the most variable of all except in the dorsal contour of the
head, and even this is aberrant in those Melinae studied.

SUMMARY

(1) On the basis of the structure of the bony auditory region and
the ossicles the arctoid Carnivora fall into three well-marked groups:
(a) the Canidae; (6) the Procyonidae, Ursidae, and Ailuridae; and
(c) the Mustelidae.

(2) The Canidae form an extremely compact group, sharply set
off from the other two groups.

(3) The Procyonidae (with the exception of Potos), the Ursidae,
and the Ailuridae are more diverse than are the Canidae, but
resemble each other more closely than they do any member of the
other two groups.

1943 AUDITORY REGION OF CARNIVORES— SEGALL 59

(4) The Mustelidae are more varied among themselves than is
either of the other two groups, but share certain features in common.
In general, the mustelid auditory structure is more distinctive than
is that of the other two groups.

(5) Potos exhibits resemblances both to the Mustelidae and to the
Procyonidae. Its auditory region is more or less intermediate
between that of these two families.

REFERENCES

DORAN, A. H. G.

1877. Morphology of the Mammalian Ossicula Auditus. Trans. Linn. Soc.
Lond., (2), 1, pp. 371-497.

KAMPEN, P. N. VAN

1905. Die Tympanalgegend des Saugetierschadels. Morph. Jahrb., 34, pp.
321-722, 96 figs.

KLAAUW, C. J. VAN DER

1931. The Auditory Bulla in Some Fossil Mammals, with a General Intro-
duction to This Region of the Skull. Bull. Amer. Mus. Nat. Hist., 62, pp.
1-352, 18 figs.

POCOCK, R. I.

1921. The Auditory Bulla and Other Cranial Characters in the Mustelidae.
Proc. Zool. Soc. Lond., 1921, pp. 473-486, figs. 14-18.

1929. The Structure of the Auditory Bulla in the Procyonidae and the Ursidae,
with a Note on the Bulla of Hyaena. Proc. Zool. Soc. Lond., 1928, pp. 963-974,
8 figs.

RIGGS, E. S.

1942. Preliminary Description of Two Lower Miocene Carnivores. Field Mus.
Nat. Hist., Geol. Ser., 8, pp. 59-62, figs. 18-19.

WILKIE, H. C.

1926. The Ossicula Auditus of the Common Badger, Meles taxus. Proc. Zool.
Soc. Lond., 1926, pp. 815-823, 5 figs.