THE OF y By ELLIOTT COUES, AM. M.D. PaD., (2 20847 © t seTam “Th GITPARAN y + y \ >» ASSISTANT SURGEON, U. S. ARMY. Se AY wo “a WITH AN APPENDIX ON THE BRAIN. By JEFFRIES WYMAN, M.D. ] {Reprinted from the Memoirs of the Boston Society of Natural History, Vol. 1, Part 1.] BOSTON : 1872. RASS epee gona ask ne te ee a ee, ot B Sn ale Ul. Tne Osrrotocgy anp Myonoey or Dipeteuys Virarntana. By Exziorr Cours. With AN APPENDIX ON THE Brain. By Jerrrres Wyman. Read October 6th, 1869. INTRODUCTORY. SoME account of the general traits of Didelphys Virginiana, its external form and covering, and dentition, may properly precede, and serve as an introduction to, the study of its bones and muscles. Among other articles upon its anatomy, physiology, and zoology, those cited below may be consulted. The present memoir is based upon original dissections and preparations, from which all the descriptions have been taken, except in a few instances where the contrary is stated ; and all the illustrations were drawn by the writer from the same sources. The opossum’s form is adapted to the execution of numerous and varied movements, but not fitted for great strength, sustained effort, or rapid progression. The animal is sluggish, usually moving slowly and deliberately ; its fastest pace is a kind of amble, when both legs of the same side are simultaneously moved, as is also the case when it is walk- ing more slowly; it has not been observed to trot or lope. It is plantigrade, and the body is usually carried so low as to barely clear the ground. The body is thickset and clumsily shaped; the centre of motion is behind the middle, as shown by the general contour, as well as by the trend of the spines of the vertebra. Extension of the body, as a whole, is less conspicuous than flexion, and is not ordinarily exhibited except when the animal is hanging suspended by the tail; but the body may be bent double, or rolled in a ball, with the greatest ease, the fore and hind feet passing by each other at the mid- dle of the belly, and the nose pointing backward, reaching the root of the tail. The faculty of bringing the fore-paws and snout into such unusual positions is undoubtedly in subservience to the necessity of certain voluntary acts on the part of the female during and immediately after the act of parturition, such as the removal of the helpless foetus to the pouch, and subsequent fostering of it. Great power of flexion is also essential to ready recovery of an ordinary position after suspension by the tail; as may easily be shown by holding up one of these animals in this position, when it will be observed to bend the neck and shoulders strongly forward, reach upward with its fore-paws until it can catch * Owen. Article Marsupialia: Cyclopedia of Anatomy North America, m, 107, pl. 66. 1851. (Description and and Physiology, m1. 1841.) Natural History.) Meras. American Philosophical Society. April, 1847. Barrp. Mammals of North America, 232. 1857. (De- (Reproduction. ) scription, ete. See other authors there quoted, for Zoology, BacuMAN. Proceedings Academy Natural Sciences, etc.) Philadelphia, pp. 40, 46. April, 1848. (Reproduction.) Owen. Anatomy and Physiology of Vertebrates, u, and AupuBoN AND BacuMan. Viviparous Quadrupeds of 111, passim. (1866-68.) MEMOIRS BOST. 800, NAT. HIST. VOL, IT, ll 42 THE OSTEOLOGY AND MYOLOGY hold of the loosely hanging hind feet; further action of all four extremities carries the paws to the root of the tail, which is firmly grasped, when the animal climbs up its tail “hand over hand” until the point of support is laid hold of; after which, by a peculiar squirming motion of the whole body, the desired attitude is attained. The same power of flexion is brought largely into play in most scansorial movements. Lateral motion of the body is also unusually free. The neck is short and thick ; its motion is chiefly that of flex- ion. The head is of moderate size, appearing in life scarcely separated from the body, reg- ularly cone-shaped, with little, if any, constriction at base, tapering gradually to a slender, elongated muzzle. The nostrils open at the sides of the extremity of the truncated, hog- like snout; the animal is reputed to have excellent olfactory powers, and the supposition is borne out by the large nasal chambers and sense capsules. The eyes, placed far up on the sides of the head, are small, with rather poorly developed lids; presumably in rela- tion to eminently nocturnal habits. The external ears are extremely large, rounded, thin and membraneous, mostly naked, suggesting those of certain cheiroptera; the sense of hearing must be judged acute, if characters of the pinna afford a criterion. The rictus is long and ample; the lips thin and scanty; the large and numerous teeth are a con- spicuous feature ; the upper canines protrude beyond the lip, and lie outside the lower lip when the mouth is closed. The opossum is pedimanous; as truly four-handed, perhaps, as many, or any of the quadrumana. The well-developed clavicles of the fore-limb, and the free coxo-femoral articulation of the hind one, permit both extremities to be widely separated, each from its fellow, and render easy the “‘ hugging’ movements necessary in climbing. Pronation and supination of the forearm are perfect; the wrist joint, besides revolving freely upon the ulna, is capable of great extension and flexion; of abduction and adduction in less degree. The fingers have much the same relative lengths as those of the human hand ; but there is less difference between them, the little finger especially being comparatively longer and stouter, and the thumb longer, less divaricating from the axis of the other digits, and less freely opposable. This condition of the thumb results mainly from the more nearly perfect parallelism and closer union of .its metacarpal bone ; still it is “ oppos- able,” in the proper sense, and may easily be brought in contact with the tip of any finger, or of all together. The five anterior digits are unguiculate; the claws are non-retractile, and therefore stout, short, blunt and only moderately curved. The member, as a whole, is to be reckoned among the more perfect hands afforded by animals lower than man. Its ambulatorial function is secondary in importance ; it finds its highest and proper use in climbing, in gathering food, and conveying it to the mouth; and in conjugal and maternal offices. The foot is nearly as much of a “hand” as the hand itself; it is equally fitted for grasping. The hallux, compared with the thumb, is even shorter, and more inclined from the axis of the other digits, to which it is freely opposable. It bears no claw ; it is club-shaped, with a rounded extremity ; in place of the claw there is a well-marked semilunar groove. The other toes are unguiculate; the claws are like those of the fingers, but longer, slenderer and more curved; the first three toes are of nearly equal lengths, and united by continuous integument beyond the first joints; the little toe is much shorter, but relatively, if not absolutely, stouter than the rest. The general shape of the foot recalls the quadrumanous type. The movements at the ankle are remarkably free, and seem to be more than simply analogous to those of the wrist. As OF DIDELPHYS VIRGINIANA. 43 to flexion and extension, the plane of the sole may sweep through an arc of more than 180°, moving from beyond a perpendicular to the axis of the leg, to beyond a parallel with the latter. The other movements of the foot, by whatever name they may be desig- nated, amount to this: that the sole may present perpendicularly inward or outward, as well as horizontally downward, by extent of adduction and abduction; and the toes may point either inward, forward, outward, or directly backward, by extreme rotation. The first two sets of motions result from the ankle joint alone, but the whole limb is con- cerned in rotation. The prehensile tail supplies a “fifth hand.” This member is from two-thirds to three- fourths as long as the head and body together; thick and stout at the base, where there is no very evident point of distinction from the body; regularly and: very gradually tapering to an obtuse tip; nearly circular on a cross-section, but somewhat flattened underneath, as if by constant pressure. The vertebral articulations and the muscles are so determined that the tail cannot be bent or curled upward, except near its base, where, also, lateral flexion is most marked; when not in use the tail is habitually carried with the tip curled under, so that it resembles a note of interrogation laid sideways e¢-. It has also a peculiar power of being twisted upon its axis, particularly toward the extremity, and so winding like a cork-screw around slender supports. The prehensile power is so great that the animal can suspend itself with only an inch or so of the tip of the tail hooked over a branch. Notwithstanding the general similarity of the long naked tail to that of a rat, it does not appear to have by its weight any special function of balancing the body when the animal is moving over uneven surfaces, as a rat’s tail is supposed to have ; its use is strictly limited to prehension; it chiefly comes into play when the animal is climb- ing, and more particularly assists it in gathering fruit from the extremity of the branches. The young attach themselves to the parent’s body by the same means. Most parts of the body are covered with two-kinds of fur. The general covering is a kind of true wool, very fine, short, densely packed, of “kinky ” fibre, imbricated and fur rowed (scaly-imbricate), and with little or no pigmentary matter. Interspersed through this fur on most parts of the body are numerous true hairs, much longer, straight, stiffish, smooth and cylindrical, uncolored at the base, but loaded with coloring matter on the terminal third or half. These hairs are most numerous above, on the sides, and about the root of the tail; but they are nowhere thick enough to hide the true fur. Below, and on the insides of the limbs the fur alone exists. The hair almost fails on the terminal third of the head; the extremity of the snout is naked. The fur extends for two inches or more on the tail; the hairs project a little further. On the extremities the fur stops at the bases of the fingers and toes, and sides of the soles and palms; but the digits are sparsely hairy. A very fine, soft, short fur clothes the scrotum and lines the marsupium. The whisker-hairs are stout, stiff, straight and very long, but not numerous. A few colorless, bristle-like hairs sparsely cover the backs of the fingers and toes, which are otherwise naked, as are the palms and soles, where the integument is thickened and variously modified, forming callosities, etc., or changing into horny plates. (Fig. 1, a, an- conal, and 6, palmar, aspect of hand: fig. 2, a, rotular, and b, plantar, surface of foot ; nat. size.) Above, the digits are covered with somewhat irregular transverse scutella, scarcely overlapping, each divided once or twice across. The palms and soles are stud- ded with small, round, convex tubercles; and the former have six, and the latter five, 44 - THE OSTEOLOGY AND MYOLOGY prominent callous pads, arranged and shaped as in the accompanying figures. The fin- gers and toes have on their under surfaces similar but better developed tubercles, to the tips, which are enlarged and rounded, and, like the pads, are not tuberculate, but present fine, curved ridges and depressions of cuticle, like the tips of the human fingers. These roughened and scabrous surfaces have evidently direct relation to the scansorial nature Fig. 1, a. Fig. 2, a. Fig. 2, b. of the animal; and corresponding modifications occur in the integument of the tail, which assumes an almost corneous condition by thickening and hardening of the epi- derm. As far as the hair extends, the skin of the tail is simply tough and dense ; beyond this, the outer covering becomes harder and divided into small plates. These are oval or subcircular, or somewhat polygonal, from mutual apposition. Pigment granules ap- pear in irregular blotches near the base; the tail is otherwise colorless. A few short, bristly, colorless hairs, like those upon the digits, lying closely appressed, are sparsely scattered over the whole tail. At any given point the scales are largest and flattest above, smallest and most tuberculate underneath ; their average size diminishes regularly, with the tapering of the tail; they are regularly disposed in oblique rows, like the dorsal scales of ser- pents; there are about thirty in a circumference ; the obliquity is slight, a row making an advance of about four scales’ length in each half turn. The reticulation is somewhat imperfectly repre- sented in fig. 8, which shows the side of the tail, of natural size, at junction of first and second thirds. Short fur ascends a little way on the back of the ear, but disappears below the meatus in front. Most of the ear is flat and ‘“foliaceous”; but toward the base it is much vaulted. The very thin lobule is of moderate size, full and rounded. The posterior margin of the pinna nearly reaches the root of the lobule, and forms a flap on the inner aspect of the latter. A crescent-formed protuberance lies just outside the termina- tion of the anterior margin of the conch, which latter is lost in a transverse fold of integument that forms the anterior or ex- OF DIDELPHYS VIRGINIANA. 45 ternal wall of the meatus, running across to meet a fold of the lobe itself. A triangular fossa is bounded by these two folds and the anterior margin of the ear. (Fig. 4.) The ear and other naked parts of the body appear to have a low grade of organization, or at least less vitality than ordinary. Under unfavorable conditions, as when the animal is closely confined for a length of time, the ears, tail and digits are susceptible of ulcera- tion, and often rapidly slough away. The Schneiderian membrane is peculiarly subject to hemorrhage; epistaxis may be brought about, for example, by the passage of a current of electricity, or by very moderate violence. The ano-genital orifice, as usually observed, has a puffy, inflated, almost prolapsed ap- pearance, as if imperfectly guarded by the sphincter: the penis is almost always, however, withdrawn. The abdominal scrotum is loosely pendulous, almost pedicellate; its livid bluish color appears through the short fur. It is capable, however, of marked contraction ; and the testicles may be closely drawn up. The pouch may be detected at a very early age ; but in the virgin, though full grown, female, it is never conspicuous, and would es- cape observation unless sought for; it appears simply as a slight duplication of the skin, capable of admitting the tips of two or three fingers. It rapidly increases with use, and attains a large size, after which, though diminishing somewhat, it never returns to its original condition. The furry lining, at first colored like the rest of the belly, acquires during the pouch-gestation a fulvous or reddish hue; the nipples, at first small, concealed in folds of the skin that are scarcely perceptible, become very prominent, and are sur- rounded by a naked aureole. There are thirteen nipples, one central, the rest disposed inacirele. The lip of the pouch can be brought very near, if not in actual apposition with, the genital orifice. The transfer of the newly-born young to the pouch has been shown to be accomplished by the voluntary act of the parent, with lips or paws, probably the former. The reproduction of the opossum, for a long time a subject of wild or fanciful specula- tion, has been satisfactorily determined by the researches of Dr. Bachman and others: to which, however, it would be out of place to more than allude in the present connection. The full history is given in the papers of this gentleman, already cited. Among the many curious beliefs that still pass current with the vulgar upon this subject, may be mentioned one that ascribes to the opossum, in consequence, doubtless, of the bifurca- tion of the penis, a coitus with the nostrils of the female; subsequent sneezing, on her part, upon the teats of the pouch causing the young to grow there. The opossum is extremely tenacious of life. It is, in fact, rather difficult to kill one for dissection without mutilation of some part. In consequence of the shape of the parts, it is not easy to reach and pierce the medulla; a better way is to open the heart with a slender bistoury introduced between the ribs. I found poisoning with cyanide of potas- sium to be the most convenient method, with the additional advantage of affording a means of studying the operation of that very violent poison; but for rapid effect a larger quantity is required than would be expected to suffice; perhaps fifty times the amount that would almost instantly kill a pigeon. The most noticeable effects are at first profuse salivation, then unsteadiness of gait and general tremulousness, succeeded by clonic spasms alternating with perfect rigidity ; ending with complete relaxation, accompanied by copi- ous alvine and urinary evacuations. The time required is from five to fifteen minutes, according to the dose, and strength of the subject. MEMOIRS BOST, 800. NAT. HIST, VOL. II. 12 46 THE OSTEOLOGY AND MYOLOGY The natural history of this quadruped has been so fully elucidated by Audubon and Bachman, that little remains to be learned. It is strictly nocturnal, like many, if not most marsupials. The females, indeed, hardly leave their retreats at all during the two months of uterine and pouch gestation. As the teeth show, it is omnivorous; any quad- rupeds or birds that it can overpower, eggs, reptiles, insects and worms, berries and fruits of all sorts, form its food; it will at times eat carrion, half a dozen individuals having been found at one time feeding on a dead cow. At certain seasons, especially in the fall, it acquires a deposit of fat, chiefly in the subcutaneous tissue, rivalling in amount that of a well-fed hog, and when in this condition its flesh is palatable. Particularly when thus fortified, it may fast a long time without inconvenience ; and under ordinary cireum- stances can survive for three or four weeks without food or water. An individual upon which I was experimenting passed this length of time with no other food than its own tail, which it gnawed off and ate during the last few days before it was killed. Accord- ing to Dr. Bachman it sometimes becomes quite tame; but all the individuals I kept in confinement remained to the last as sullen and ferocious as when first caught. It has no voice beyond a low growl, and a sort of hiss when provoked; under the influence of fear or anger a transparent viscid saliva drops constantly from the mouth. Its personal habits, in close confinement, are filthy; the odour, which under the most favorable circum- stances is not agreeable, becomes very disgusting after a time. To a common observer it appears dull and uninteresting in confinement, passing the day huddled in a corner, and the hours of darkness in persistent endeavors to escape. It ordinarily, or at least for a long while after capture, refuses food by daylight, and resents too familiar approach by a sinister expression, and the display of a formidable set of teeth. If fortitude under physical pain, and acceptance of death without dismay, be tests of courage, the opossum is one of the bravest of beasts; no amount of torture can draw from it a sign of suffer- ing. Such endurance, joined with remarkable cunning, enables the animal to feign death so perfectly that it frequently escapes, and not seldom retaliates upon its persecutors with an unexpected wound from its sharp teeth. These are well known traits, that have given to the English language a proverbial expression of peculiar significance. THE TEETH. Dental Formula : —Ine. 74; can. 53 prem. 3-3; mol. #4; = % = 50. As the foregoing formula shows, four kinds of teeth are present, and indicate an omniy- orous animal of carnivorous propensities. In number the lower teeth only differ from the upper in being one less on each side, by the absence of one pair of incisors. In size and shape, however, the upper and under teeth, especially the molars, differ considerably. The dentition apparently comes nearest to that of Thylacinus cynocephalus, among Didel- pha, and thus resembles that of the Canide. (For general comparisons of the teeth with those of other marsupials, see Owen, Comp. Anat. and Phys. Vert., m1, p. 285 et seq.) Upper Jaw. The median pair of incisors are longer than the rest, and stand a little apart from the succeeding ones. The others are in close juxtaposition, though not usually in actual contact; and of the same size. All are simple, straight, short, somewhat com- pressed, with slightly enlarged head, not pointed or edged, but obliquely truncated, ee en OF DIDELPHYS VIRGINIANA. 4” resulting in a flattened, cutting surface that looks forward and inward. The long axis of the tooth is almost vertical: the root is unicuspid and rather longer than the exposed portion. An average incisor, root and all, is a fourth of an inch long. A considerable interval occurs between the incisors and the canine, for the reception of the lower canine. The upper canine is of remarkable size; this formidable weapon is as large, comparatively, as—even if it does not exceed—the tusk of the Felid@ or Canidae. The whole tooth is over an inch in length, in an adult male; rather more than half its length protrudes from the socket. In the female, the tusk is both relatively and abso- lutely smaller. The large root causes a protuberance of the jaw-bone along its course ; the base is abruptly truncate, with so large a cavity that the rim of the opening is a thin, sharp edge. The canine is compressed laterally and curves regularly from base to tip in the are of a large circle; so, although the course of the root in the alveolus is obliquely forward, the point of the tooth is directed downward, and a little backward. In an un- worn condition, the tooth tapers to an extremely sharp point, which reaches below the edge of the mandibular alveolus. The opossum’s bite is severe, and inflicted mainly by the four canines, which the animal can easily thrust through stout boot-leather. The first premolar is much smaller than the other two, from which it is separated by a wide interval. It is inserted immediately behind, in juxtaposition with, the canine, and almost appears like a basal snag of the latter. It has two very long, slender, parallel roots, curving slightly backward; these roots do not join to form the body of the tooth until fairly outside the socket; a slight groove is continued downward from their point of union. The exposed portion of the tooth is very small—scarcely half as long as the root; it tapers rapidly to a single sharp point; with no noticeable basal dilation. The second and third premolars successively enlarge; the exposed portion correspondingly preponderates over the rooted part; the latter is deeply cleft into two prongs as before, but these are stouter, straighter and divaricating a little from each other. These teeth are unicuspid, like the first premolar, but heavier, blunter, with a thicker base; and at the base of the posterior border a well developed snag is given off. The second premo- lar touches the third by means of this basal snag: the third is opposed to the first molar by the same means. The first three molars are very similar to each other in essential characters, and in- crease in size from first to third; the fourth, or wisdom tooth, is smaller than either of the others, and differently shaped. It is cut later than the rest; I find it scarcely pro- truding from its socket in an animal two-thirds grown. The cutting of all the molars appears from before backward. The molars are in mutual contact by their external borders; the triangular shape of the crown produces a large interval between contiguous teeth along the imner border. The crown of the first three molars has the shape of a right-angled triangle, with nearly equal base and perpendicular. The anterior border proceeds straight inward toward the median line of the mouth; thence with a rounded termination obliquely backward and outward to meet the external border. Each of the three corners of the teeth developes a moderate cusp: the middle of each side rises into a smaller tubercle (that of the oblique side being the largest); the grinding surface surrounded by these elevations is irregularly concave. On viewing the molars from the outside, each appears almost divided into two by a deep notch in the lower border, which is opposite a similar notch in the upper border, formed by the late confluence of the fangs. 48 THE OSTEOLOGY AND MYOLOGY The molars have three roots, corresponding to the three corners of the crown; two of the roots are side by side, and external; the third stands alone; all are straight, slender and nearly parallel; each prong enters the alveolus by a distinct perforation of the bone. The last molar has three roots like the others, but these are differently disposed. The outer one stands alone, and is much larger than the other two, which are placed side by side, though they still enter the alveolus by separate apertures. The surface of the tooth, instead of being triangular, represents half an ellipse divided along its major axis; the curved border is posterior; the straight one is nearly transverse, like the corresponding anterior border of the preceding molars. The crown of this tooth presents an elevated rim and a central depression, divided into two halves by a raised line that traverses the short diameter. The inner and outer angles of the two parts have slightly developed cusps ; the middle of the straight border rises in a small point. The surface represented by the conjoined crowns of all the molars is but little convex in the longitudinal direction; the point of greatest convexity is at the penultimate molar, whence the curve sweeps rapidly up past the last molar, and more gradually up along the anterior ones. The most promi- nent molar just reaches a line drawn from the tip of the canine to the apex of the paroc- cipital. The grinding surface of the last molar looks directly downward; that of the others is more and more obliquely bevelled from third to first, so that the grinding surface of the conjoined molars is twisted upon itself. The transverse diameter of the upper molars contrasts strongly with the narrowness of those of the lower jaw. (For upper teeth, see fig. 10 and fig. 11, p. 64.) Lower Jaw. The four incisors on each side repeat the main characteristics of those of the upper jaw, but the median pair are not noticeably longer than the rest, nor do they stand apart from the others. These teeth are in close proximity to each other, but not touching. The chief peculiarity is their great forward and outward inclination; the an- terior ones, in fact, are almost horizontal, instead of vertical. When the jaw is closed, the four teeth are opposed to the interspaces between the four lateral upper incisors, falling inside the line, and not reaching the median pair of the latter. The lower canine is smaller than the upper, but still of great size. It is inserted in juxtaposition with the last imcisor, and closes in front of, and inside of the upper canine, in the wide space that occurs between the latter and the upper incisors. Its root is longer than the exposed portion, much flattened, and inserted very obliquely in the bone. The free portion is stout at the base, rapidly tapering to a sharp point, and much curved, to bring the tooth into the vertical position from its very oblique line of insertion. Asa whole, the tooth is notably twisted upon its axis. It measures, in adult examples, nearly an inch in length. The first premolar is situated midway in the wide interval between the canine and the second premolar. This brings it directly under the upper first premolar, and leaves room for the apex of the upper canine to descend between it and the lower canine. This tooth repeats the characters of the corresponding upper one, but is still smaller. The second is much the largest of the under premolars, but scarcely differs otherwise from the third, with which it is in contact by a largely developed basal snag on the posterior border; a similar snag upon the third connects the latter with the first molar. These teeth close in advance of the corresponding upper ones; so that the second premolar passes by its pos- terior border along the anterior border of the second upper one; the latter is wedged OF DIDELPHYS VIRGINIANA. 49 between the second and third under premolars, its apex resting upon the basal snag of the former; the third under premolar is similarly wedged between the second and third upper ones; the latter, again, fits in the interspace between the third under premolar and first molar, with its apex touching the basal snag of the former. Each of the premolars has two long, stout, straight roots, parallel with each other; they cause a deep sulcus to appear on both inner and outer aspects of the exposed portion of the tooth, before unit- ing to form the single, compressed, conical cusp. The anterior border is a little convex ; the posterior twice concave, as a result of the projection of the basal snag. The four molars are remarkable for possessing but two roots apiece, like the premolars, instead of three as those of the upper jaw have. In the formation of the crowns, how- _ ever, the transition from the premolars is abrupt and decided. The roots are straight, slender, terete, tapering, diverging but little from each other, following each other in the line of the axis of the jaw-bone, and are two or three times as long as the exposed por- tion of the tooth. The contour of the crown corresponds to the absence of a third (inner) root; it is a narrow rectangle, without angular extension inward. While quite as long as the upper ones, each for each, the lower molars have not half the breadth of the latter; when the two sets are apposed in the closed jaw, a considerable part of the grind- ing surface of the upper molar lies external to the under series. The molars progres- sively increase in length, width, heighth and prominence of the several irregularities of surface, from first to fourth. The antero-external corner of each presents a concave depression for the reception of the cusp upon the posterior border of the corresponding upper tooth; a larger excavation upon the face of the crown, at its back part, similarly receives the prominent inner corner of an upper molar. Between these two depressions the outer border of each molar rises to form a prominent conical cusp, attaining the great- est elevation on the last of the series. Behind the cusp is a smaller one, at the postero- external angle of the tooth; the postero-internal corner makes a still less prominent cusp. The antero-internal angle rises highest of any; its strong cusp is partially divided in two by anotch. (For under teeth see figs. 7 and 8, page 62.) The upper molars have much more of the character of true “ grinders” than the lower ; the latter are evidently better fitted for the laceration of animal tissue than for the crush- ing of vegetable fibre. The crowns of the upper molars chiefly present obtuse rounded inequalities of surface; those of the lower show conical sharp-pointed projections. _* The whole series of lower teeth is twisted upon itself, and traverses the jaw obliquely, as shown in fig. 8. PART I.— THE BONES. 1. Tue SKULL. The cranium is to be considered under three different aspects. First, as being a phase of the mammalian modification of what has been called the vertebrate archetype ; repre- senting, as such, four vertebrae, the elements of which have been subjected to changes of relative size and shape, and also, in many instances, to dismemberment or displacement, and the essential characters of which have been further modified — not to say obscured — by the addition or intercalation of accessory and appendiculate parts, as well as by the MEMOIRS BOST. SOC. NAT. HIST, VOL. II. 13 50 THE OSTEOLOGY AND MYOLOGY suppression, or at least the want of distinctness, of certain parts found in those crania that depart less from the primitive type. Secondly, as being composed of numerous individual parts or pieces; that is, of separate bones, suturally joined with contiguous ones, but not continuous through codssification ; such parts expressing not only the divi- sion of the skull into four vertebral segments, but also the distinction of the several ele- ments of each segment, and of its appendages. Thirdly, as a complex whole, resulting from the coherence or confluence of all these several proximate and ultimate parts; as such forming a bony box to contain and protect the brain and organs of the special senses. From the first view may be learned the ultimate, or true, or de jure morphology of the skull; from the second, its proximate or apparent, or de facto morphology; from the third, its teleology, which is the aim and end of all morphology. The cranium, guoad four vertebre, represents or typifies the skull of all vertebrates; as to its individual bones, the skull of all mammals ; as a whole, only itself—so far is general morphology modifiable for special teleology. No two leaves of the forest are precisely alike; no opossum’s skull is an exact dupli- cate of another’s. One skull may be so described that the description will not apply to any other one ; or in such a way that the terms employed will indicate the skull of any vertebrate ; and each of the two methods are perfectly truthful. An account of individ- ual peculiarities only would be tediously profitless ; a summary of class, ordinal, or fam- ily points alone, would leave much to be desired; a description of specific characters is alone satisfactory, for these mark one entity amidst innumerable forms of animal life. Specific description of a skull may be more general or special, according as more or fewer relations are considered. In a general way, the relations of the opossum’s skull to the crania of other vertebrates may be indicated in a description of its four vertebree ; to that of other mammals, in a notice of its different bones; and to that of other marsupials, in its consideration as a whole. It suits my purpose to take up these three lines of de- scription, and in the order just indicated. a. Of the skull as four vertebra (diarthromeres A), V. epencephalica.—The hxmal arch rests upon the thorax ; the centrum and neural arch can usually be detached entire from the antecedent vertebra, without disturbing the latter. The only element that ever codssifies with the antecedent segment is the neural spine, and this confluence only takes place in aged skulls. The neural spine is rarely consoli- dated with the neurapophyses; the latter, though meeting above the foramen magnum to complete the neural canal without the intervention of the neural spine, remain distinet from each other, but early unite with the centrum. The parapophyses even sooner unite with the neurapophyses ; coussification is completed in specimens that still show traces of separation between neurapophyses and centrum. In physical characters, the occipital 1“ Diarthromere.— A word needed to supplant ‘ vertebra,’ in the sense attached to this last in this paper. ‘Vertebrate’ may be sufficiently definitive of a back-boned animal, but ‘vertebra,’ at best a meaningless name of certain ‘bones,’ is inadequate to express the required idea of a whole segment of an animal constructed upon the double-ring plan, besides being so firmly contracted by custom that the necessary ex- pansion is undesirable, if not impracticable. The typical segment—zoonule, or somite—of single-ring animals (Articu- lates) is already called arthromere, of which the proposed word is an obvious analogue.’’—New York Medical Record, Dec., 1870. Although we should of course prefer to employ this term, which we recently proposed, as above, yet it is now prac- tically impossible to revise the ms. for this purpose. OF DIDELPHYS VIRGINIANA. 51 segment, as a whole, is not more unlike the succeeding one — the atlas— than the latter is unlike the axis; or than this last is different from a coccygeal, for instance. Aside from the displacement of its hamal arch, its chief modifications relate to its firm sutural (instead of loose articular) connection with the antecedent vertebra, and to the expansion of its neural arch to accommodate the epencephalon. Taking away the ex- panded neural spine, the three remaining elements form an osseous ring very similar in general appearance to the atlas. It does not appear that the sum total of the modifica- tions that this vertebra has undergone in becoming a part of the “skull” instead of the “spine,” is greater than that suffered by the contiguous cervical one, which latter is none the less a “ vertebra” for wanting centrum, neural spine and hemal arch ; and it is cer- tainly less than that characterizing the last coccygeal,—a bone that has nothing of a vertebra but a degenerate centrum, fails to perform the essential office of a vertebra, and is only recognized as a vertebra in virtue of its location, being to all purposes, as well as appearances, the distal phalanx of a digit. Comparison of the two ends of the chain of vertebral links shows that teleological modification is less obscure in the guise of hyper- trophy, than under the mask of atrophy; exaltation of the function of parts may entail less change than degradation ; the caudal larval vertebra deviates more from the arche- type than the cranial imago does. V. mesencephalica. Centrum and neurapophyses coalesce with the same elements of the antecedent (frontal) segment; the compound bone so formed (“sphenoid’’) is simpler than in some mammals, from not including an appendage of the maxillary arch; for the pterygoid is a distinct bone, as in birds. The neural spine is large, coincidently with increase in size of the ides anions though it is itself surpassed by the frontal spine. It is developed from two centres, and the lateral moieties so long remain discrete, that they are so found in the great majority of adult specimens; but they ultimately fuse together; they never codssify with either frontal, squamosal, or sphenoid ; they reach their proper neurapophyses. ‘The parietal parapophyses (mastoid) are disconnected with that arch, coussified with the acoustic capsule, and wedged in between the latter, the occipital neur- and par-apophyses and the distal element of the appendage of the palato-maxillary arch, which appendage serves as a strong osseous bridge connecting the hemapophysis of the nasal vertebra with the par- neur-apophysis of the ultimate and penultimate cranial seg- ments, and slightly joins, also, the neural spine of the occipital segment. Besides the extensive confluences and sutures that the parietal segment presents, it also contains — 2. €., is the site of — the frontal pleurapophysis, — the proximal element of the mandibular arch (tympanic). This, however, has no osseous connection with the rest of the skull; its relation to the parietal segment is merely one of position. The proper pleurapophysis of the parietal segment (stylo-hyal) is wanting; the other elements of the hyoidean arch! * The os hyoides may be conveniently noticed in this con- ported upon the basi-hyal, but also touch the thyro-hyals. nection. The size and general shape are shown in the The basi-hyal is small; as high as wide; irregularly five-sided, accompanying figure (fig. 5). The five or rather sub-quadrate, with a thickened, upright, posterior, pieces comprising the hyoid are usually lower border between the ends of the greater cornua, spread- (always?) distinct: I have never seen ing obliquely upward and forward into a thin lamina with them anchylosed. The thyro-hyals are _ slightly thickened sides, supporting the lesser cornua, and elongate, much compressed in an ob- _ having its sharp, free anterior border somewhat produced by Fig. 5. lique plane, enlarged at either extrem- cartilage. There is no uro- or glosso-hyal; no lytta could be ity, slightly curved in the continuity. found in the substance of the tongue. The cerato-hyals do The cerato-hyals are small, flat, triangular nodules remaining not appear to have ever formed two pieces. cartilaginous for a considerable time. They are mostly sup- 52 THE OSTEOLOGY AND MYOLOGY are only connected with the skull by soft tissues, and are reméved from its immediate vicinity ; the styliform occipital parapophyses appear to perform, as far as muscular at- tachments are concerned, the office of the hyoidean pleurapophyses. V. prosencephalica. ‘The confluence of the frontal centrum and neurapophyses with those of the parietal segment, has just been noticed; these elements are distinct from their homotypes of the nasal vertebra. The neural spine, at first in lateral halves, speedily consolidates into one piece, but codssifies with none of the numerous surrounding parts. It is remarkable for its size, and for taking greater part in the formation of the olfactory chamber, than in the protection of the prosencephalon. Frontal parapophyses are wanting; at least no post-frontals can be demonstrated, nor is there evidence war- ranting the assumption that they are included, otherwise than theoretically in the frontal, or a neighboring, bone. It may however, be surmised, that the super-orbital protuberances of the frontal bone may represent, teleologically at least, such para- pophyses ; since they subserve the usual function of limiting the orbits posteriorly. The pleurapophysis is disconnected with the other elements of this vertebra, dislocated, lying between the last and the penultimate vertebra, and pressed into the service of the organ of hearing ; it is a delicate little half-ring, supporting the membrana tympani and ossicula audittis. As already noted, it has no osseous union with the rest of the skull; and even its membranous attachments are of the loosest nature. No trace of separation of haema- pophysis from hemal spine is usually to be observed; nor is there any distinction of the several hemapophysial elements that remain distinct in lower classes, and may theoreti- eally be taken to exist in the present example. The hamal spine is permanently sepa- rated into halves, as far as osseous symphyseal union is concerned. The frontal hama- pophysis not only articulates with the distal element of the maxillary appendage, as usual inmammalia, but also with the proximal element of the same; and moreover—what is a rare modification — with the parietal neurapophysis, (squamosal, malar and alisphenoid thus conniving to form the glenoid cavity); it is not connected with its own, or other pleurapophysis. V. rhinencephalica. The centrum, permanently distinct from that of the succeeding vertebra, is an attenuated osseous spiculum, only having solidity behind for articulation with the presphenoid ; it is in intimate relation throughout with the enormously devel- oped olfactory sense-capsule. The neurapophysis, if it have any actual existence, is ob- scure, and not to be satisfactorily demonstrated in any part of the olfactory capsule; but it may be conjectured that the vertical “septum nasi” represents coalesced pre-frontals. The neural spine is permanently bifid, and remains distinct from surrounding bones; like its centrum, it has been subjected to longitudinal extension and attenuation, to close in the upper wall of the conical muzzle. This vertebra has no diapophysis. The pleura- pophysis forms but little part of the maxillary arch, though still performing its proper office much more notably than does the same element of the frontal vertebra. As usual, it is curiously distorted in shape, and closely wedged in between numerous elements not of its own, but of two succeeding, vertebrae. The development of the nasal hamapophy- sis corresponds with that of the frontal ; it articulates not only with its fellow, its pleura- pophysis, and its hemal spine, but also with the neural spines of its own and of the suc- ceeding vertebra; it likewise supports, in part, a mucous scale-bone and convolutions of a sense-capsule ; and besides is connected, through the intermedium of two accessory appen- OF DIDELPHYS VIRGINIANA. 53 dicular pieces, with the neur- and par-apophysis and neural spine of the parietal, and neu- ral spine of the occipital, vertebra. So the nasal hamapophysis connects mediately or immediately, its own with each of the three other cranial vertebrae ; such extensive con-— nections not only contribute to the strength and fixity of the palato-maxillary arch itself, but of the whole skull. The hxemal spine is permanently double; it maintains distinctness from the hemapophysis; above, the apex of the neural spine is wedged in between its lateral moieties ; below and anteriorly, these are in mutual apposition, thus closing in the termination of the neural canal. The “ diverging appendage” of the palato-maxillary” arch (pterygoid), a delicate scale-like bone, loosely attached, extends backward under the centra of the frontal and parietal arches; it falls far short of the frontal pleurapophysis, instead of connecting the latter with the nasal pleurapophysis, as in birds; nor is it sub-~' servient in any way to either the stability or the mobility of cranial segments. The or- bital dermo-skeletal, and the acoustic and olfactory splancho-skeletal, bones are elsewhere noticed: the opthalmic sense-capsule is unossified. . b. Of the Several Cranial Bones. Most of the cranial bones of this low mammal retain their individuality; a few others only codssify with age; several are confluent from an early period of their formation, if not originally connate. The skull is readily disarticulated, and reducible to a greater number of pieces than is the case with higher mammals. Nearly all the bones afford in- structive evidence of their morphological characters. The comparatively few compound bones result from confluence of but few morphologically distinct elements: there is no such complex “bone” as, e. g., the “sphenoid” or “temporal” of anthropotomy. The mammalian modification of cranial vertebrae may therefore be studied under advantageous circumstances. The skull ordinarily falls into the following pieces, some of which are simple, and others compound, bones ; the latter are in italics :—occipital ; superoccipital ; two petromastoids ; parietal, either single or double; tympanic (with ossicula audits attached) ; sphenoid ; vomer with ethmo-turbinal ; frontal, single or double ; two nasals ; two premaxillaries; two maxillaries; two palatals; two pterygoids; two malars; two squamosals; two lachry- mals; and the mandible (qu. italics), in halves. Sometimes the great sagittal crest comes off the parietals, having only sutural union, and being apparently developed in the fascia between the temporales. The superoccipital may unite with the parietals in old age. The interparietal suture is often obliterated ; the interfrontal usually. There are no pre- or post-frontals, nor stylo-hyals; unless the vertical plate of the ethmoid represents the first named. I have not observed an interparietal, or Wormian bones. Ordinarily there is nothing to indicate that the mandible is formed of articular, angular, splenial, coronal, and dentary pieces; but its symphysis is imperfect, being only a synchondrosis, like the pubic and sacro-iliac. The following are the commonly observed confluences of originally distinct bones :—Ex~ and par- with basi-occipital, producing an “os occipitis” the spine of which remains sepa- rate ; its neurapophyses meet above the foramen, but do not ordinarily coalesce. Bast-, ali-, pre- and orbito-sphenoid, resulting in an “os sphenoidale” which is not further compli- cated by codssification with the pterygoids. Mastoid with the otic sense-capsule (petrosal) forming the “ petromastoid”’; the separation of these elements from the squamosal and tympanic, and the absence of a stylo-hyal, reduces the ordinarily complex “os temporis’”’ MEMOIRS BOST. 80C, NAT. HIST. VOL. H. 14 54 THE OSTEOLOGY AND MYOLOGY to almost its simplest expression. Vomer with the olfactory sense-capsule (ethmo-turbi- nal); but the codsification is not perfect throughout. A spongy bone (“inferior turbinal’”’) is attached to the inner surface of each maxilla. The special characters of each of the bones above mentioned are now to be examined. The basioccipital is only seen as a distinct bone in very young animals, although traces of separation from the neurapophyses persist during adolescence. Thus I find the suture, in an animal three-fourths grown, extending from the side obliquely inward and backward to the condyles, and exhibiting the part taken by the bone in the forma- tion of the latter (about one third). The precondylar foramen is developed in, or just to, one side of this suture; another smaller opening exists a little internal to the last. The connection of the bone with the petrosal is slight; the most marked portion of the foramen lacerum posterius is just in front of the paroccipital. The forward extension of bone is simply a flattened plate, nearly four-sided, but tapering a little, abutting by a straight, transverse suture against the body of the sphenoid ; longitudinally ridged under- neath, smooth and slightly concave above, with an oblique groove alongside each anterior corner. The exoccipitals rise from the back part of the body, develop the superior two-thirds of the condyles, and close over the foramen magnum, where, however, they do not ordina- rily coalesce; the line of separation remained in all the skulls examined. They form a broad arch, whose longitudinal expansion is oblique, perforated on either side, at the base of the paroccipitals, and notched in the middle of the upper border for a de- scending process of the superoccipital; sometimes the upper border has a well marked prominence indicating the point where the superoccipital and mastoid come together. I have never found the paroccipitals distinct from the exoccipitals. They form pointed conical processes of moderate length, directed downward, almost parallel with each other, roughened for muscular attachment. Their bases abut against the petromastoids; a deep notch separates them from the condyles. The superoccipital is rarely, if ever, confluent with the exoccipitals. 1 have never seen it thus united. On the other hand, its bony union with the parietals is fre- quent, and with the “temporal bone,” at the junction of the mastoid and squamo- sal, is sometimes seen. The lambdoidal suture, when not obliterated, is very irregular in direction, and mostly squamous in character, the outer surface of the superoccipital being bevelled for some distance. The superoccipital is a triangular pyramid, with an excavated base and sides: the concavity of the former being for the accommodation of the brain, and of the latter for muscular attachments. The excavations of the sides are virtually produced by the elevation of the occipital crest, which forms a segment of a circle from side to side; and by the beginning of the sagital crest, which runs directly forward from, and at right angles with, the middle of the transverse crest. The thickness and solidity of the bone, however, is not entirely owing to these crests. Confluence of the basi-, pre-, ali- and orbito-sphenoid takes early place, and, when per- fected, results in the single complex bone ordinarily called by the collective name “ sphe- noid.” I find no traces of separation of these four bones in the youngest specimens examined. As might be expected from its composite nature, the connections of the bone are numerous. The contiguous bones are :—behind, the basisphenoid, and more OF DIDELPHYS VIRGINIANA. 55 laterally, the petrosal and squamosal (which separate it from the mastoid), and sometimes the malar: on either side, the parietal and frontal: before, the vomer, ethmo-turbinal, palatine, and pterygoid. There is no actual connection with the tympanic; the mastoid is still further removed. I have never found it confluent with either of these con- tiguous bones: its connection with the palatines is perhaps most intimate; with the pterygoids and petrosals loosest. Its frontal and squamosal sutures are the most exten- sive ; but that with the parietal is of considerable length, since the squamosal does not lift the parietal far away from the sphenoidal ala—though it is difficult to say how much of the spheno-parietal suture is formed by the orbitosphenoid, and how much, if any, represents the union of the neural spine with its proper neurapophysis, the alisphenoid. The basi- sphenoid alone unites with the basioccipital: the alisphenoid alone with the petrosal and squamosal; the orbitosphenoid with the frontal. Junction with the palatines is through three of the elements of the sphenoid: the ali-, orbito- and pre-sphenoid ; union with the former being effected by the “ pterapophyses”’ to be presently noticed: the sphenoidal relations of the pterygoids are almost wholly brought about by the same means. The articulation of the sphenoid with the basioccipital and vomer respectively is simple abut- ment of the thickened, squarely truncated extremities of these bones; that of the pala- tines and pterygoids is the lateral apposition of elongated processes ; that with squamosal, parietal and frontal is the true, squamous suture, most decided in the latter case, where the orbitosphenoid very extensively overlaps. When the alisphenoid reaches the malar at all, it is a simple touching of the tip of its slender, lateral, articular process. Junction with the petrosals is imperfect and irregular. The compound “ sphenoid bone,” thus made up and related, presents a stout, flattened “body,” tapering from behind forward, distinguishable from the “ wings” by its shape and solidity, and separated from them by grooves, ridges and foramina, apparent both above and below. The under surface is smooth; on either side are seen, from behind forward, the termination of the foramen Jacerum, medium: two foramina looking forward and out- ward, separated by a slight bony bridge, the posterior opening leading backward by a groove into the foramen lacerum, and forward into the sella turcica: the anterior, which is the larger, curving inward and communicating with its fellow below the sella: the groove sep- arating the pterapophyses completes the lateral boundary of the body. Inside, the body presents a deep hemispherical or hemiellipsoidal excavation, the sella turcica, on either side of which (but really in the ala of the bone) is the entrance of the foramen rotundum, now a long canal, that opens just outside the root of the pterapophysis. More anteriorly the body is bounded on either side by a deep semicylindrical canal, which conducts the optic nerves out through the foramen lacerum anterius. The alisphenoidal part of the bone stands away from the body, on nearly the same horizontal plane, having but little upward inclination; and supports three very noticeable processes. The most posterior of these is a moderately large bulla ossea, or conch, in sub- serviency to the acoustic organ. It is a hemiellipsoid in shape, its convexity looking for- ward and downward, its large opening directly towards the tympanum. It is small in size compared with that occurring in some marsupials. Besides its physiological relation to the organ of hearing, it serves to complete, internally, the irregular parapet surrounding the fossa in which the delicate tympanic hangs suspended: the other minarets being the par- occipital, mastoid and zygomatic process of the squamosal. The petrosal lies along its 56 THE OSTEOLOGY AND MYOLOGY inner side ; its outer side is in relation to the glenoid cavity and condyle of the jaw ; at its inner base isa large foramen, sometimes, however, only a groove continuous with the foramen lacerum, from deficiency of the bony trabecule completing its periphery. Perhaps the most interesting point connected with the sphenoid bone is the share it takes in the formation of the glenoid cavity, by means of a long, sharp-pointed “ artic- ular” process of the alisphenoid, which runs along the inner and anterior margin of the glenoid cavity, and usually reaches to the malar. This formation decidedly augments the transverse diameter of the glenoid, and a considerable part of the mandibular condyle plays upon the expanded and slightly concave under surface of the sphenoidal process. The- latter, however, lying wholly in the general plane of the joint, does not afford an anterior wall of abutment for the condyle, like that furnished behind by the downward extension of the squamosal. The extent of the alisphenoid thus appropriated by the squamosal, per- haps explains why so much of the sphenoparietal suture is made by the orbital plate of the bone alone. This structure of the glenoid obtains in many, if not most, marsupials. Anteriorly the sphenoid sends off on either side a long attenuated pterapophysis, which diverges from its fellow as it proceeds forward. These slender prongs reach as far for- ward as the termination of the bony palate; forming the main connection of the ptery- goids with the sphenoid, augmenting the connections of the latter with the palatines, and forming part of the wall that laterally bounds the vestibule of the posterior nares. Absence of a stylo-hyal, and non-confluence of the petromastoid, leave the “ temporal bone” to be represented by the sguamosal alone. There being but a small vacuity in the cranial wall to be filled by the squamosal, the greater part of this bone is devoted to the formation of the back part of the great zygoma, and the support of the mandible. The zygomatic process at first stands out at right angles with the squamous portion and then curves forward, expanded vertically, to effect extensive overlapping suture with the malar. The root of this process is thickened transversely for the site of the glenoid cavity; it sends downward a stout process to form the posterior wall of the articulation, and presents inwardly a rough, irregular projection for the alisphenoidal suture. The squamous part of the bone takes but small part in the formation of the wall of the encephalon, and less than appears upon viewing the skull outwardly, in consequence of the extent to which it overlaps the parietal. The posterior border of the squamosal is somewhat thick- ened and rises in a prominent ridge, which is the lateral downward continuation of the semicircular occipital crest. The termination of this crest represents the postero-lat- eral corner of the skull, in which the mastoid scarcely takes part, being small, and wedged in between the squamosal and the ex- and par-occipitals; separating the former from the last two. Above, the squamosal comes in direct relation with the super-occipital. A prominent ridge from the upper border of the zygomatic process connects the latter with the squamo-occipital crest, and overarches the otocrane. The fossa in which the parts of the ear lie is represented, in the lateral view, by a wide and deep notch between the glenoid process, and the termination of the occipital crest. Two large foramina are situated in the notched part of the squamosal. A well marked groove, extending along the inner aspect of the back part of the squamosal, is converted into a canal (venous) by a corresponding groove in the surface of the closely apposed petromastoid. Confluence of the “mastoid process” with the otic sense-capsule results in the petro- mastoid, a small, hard, very irregularly shaped bone, firmly wedged in between the OF DIDELPHYS VIRGINIANA. 57 alisphenoid and basi-occipital before, the squamosal and ex-par-occipital behind, with an anterior prolongation that just touches the basisphenoid, dividing the fissure lacer pos- teriores et medi from each other. It always remains distinct from the surrounding bones. When in situ, but little appears on an outside view ; viewed from within, it is seen to take a large part in the wall of the brain-case, presenting a smooth, sub-triangular surface, in- dented with a very deep, conical, cerebellar fossa, below which is the eonspicuous opening of the internal meatus. Most of the superficies presents a very irregular roughened sur- face, closely appressed to the thickened root of the squamosal, with a venous canal running across it. The part that lies external, between squamosal and occipital, is smoother, being only slightly roughened for muscular attachment. The petrosal part proper appears, viewed from below, as a small, tapering piece of bone, with a swollen base, this convexity expressing the dome of the “whispering gallery’ into which the interior is sculptured. But the description of the internal structure of the bone rather pertains to an account of the organ of hearing, than to the subject of the present memoir, and must therefore be omitted. The tympanic is a delicate little oval ring, incomplete at part of its circumference, like a horse shoe. It is not bullous, and has no bony attachment with the rest of the skull. It supports the auditory ossicles, one of which is anchylosed with it. There are no decided angles or processes; but a noticeable groove for the membrana tympani, and another representing the confluence of the ossicle; the latter adds considerably to the tympanic half-ring, strengthening the slender, spicula-like prong; it is applied to the outer periphery, terminating by a somewhat thickened, rounded extremity ; at the free end bent abruptly upon itself, in the plane of the tympanum, and extending, as the handle of the malleus, almost across the space to the back of the ring, as an extremely delicate spicula. The parietals finally coalesce with each other along the median line; but in most specimens the sagittal suture is retained. The contour of the bone is very irregular; all the sutures except the sagittal are strictly squamous. The anterior margin overlaps the frontal bone for a great distance; the posterior, the super-occipital, but to less extent; conversely, the lower border is overlapped by the sphenoidal ala and the squamosal. View- ing the articulated skull from the outside, the parietal seems only joined below by the orbito-sphenoidal and squamosal plates; but, on removal of the latter, the posterior portion of the margin is seen to effect junction with the alisphenoid itself. At least, such is my impression from consideration of the parts of the sphenoidal ala, most prop- erly to be regarded as ali- and orbito-sphenoidal; for the spheno-parietal suture extends to the base of the sphenoidal process that enters into the mandibular articulation. The parietal bones are thin— almost diaphanous; internally they exhibit a decided hollowing, whose concavity is greater than the outside convexity of the bone; the meningeal vessels impress well marked grooves along the posterior border of the vitreous table; the falx cere- bri a single longitudinal one along the median line. The inner table of the bone is of much less extent than the outer, in consequence of the various extensive squamous sutures of the latter : there is little diploé between the two; its texture is coarse, with large interstices. There were no inter-parietal bones, nor ossa Wormiana, in any of the specimens examined. Even in young individuals, in which the sagittal suture is perfect, there ‘exists only a trace of the frontal suture; and this is speedily obliterated. The confluent frontals, taken as one bone, present a remarkable shape. Only a small part is devoted to the pro- MEMOIRS BOST. SOC.NAT, HIST. VOL. II. 15 58 THE OSTEOLOGY AND MYOLOGY tection of the brain; the rest forms the roof of the nasal, and the inner wall of the orbital, cavities. The point of greatest constriction of the brain-case is in the frontal bone, just in advance of the overlapping of the border of the parietal. The greatest width of the cranial] part of the bone is much narrower than the nasal part ; its depth se equally as much legs, While the cranial cavity of the bone will barely admit the tip of one’s forefinger, the nasal will contain nearly the whole of the first segment of the thumb; the former is nearly cylin- drical ; the latter represents the frustra of two cones placed end to end; the strait between the cranial and nasal cavities is perfectly circular. The coronal suture, never obliterated, may be either directly transverse, or obliquely placed, inclining forward from above down- ward; it has no notches, or other notable irregularities along its course. The coronal, and all the other sutures around the frontal are strictly squamous ; the several bones (parietal, orbi- to-sphenoid, maxillary, palatal, lachrymal and nasal) all overlap the frontal, except for about half of the fronto-palatine suture, when the reverse takes place. The outer lower margins of the bone are very irregular in their course, but in general terms, may be said to be direct from the lower corner of the coronal suture through part of the palatal, and thence to extend obliquely upward and forward to the point of greatest anterior extension, between which point and its fellow there is a very deep notch for the reception of the nasals, usually ex- tending quite to the supra-orbital protuberance. The anterior part of the surface of the frontal is perfectly smooth, and nearly flat in every direction; it is bounded in front by the naso-frontal, and at the side by the fronto-lachrymal, suture. Behind, it is separated from the orbital and temporal surfaces by two prominent ridges, which, arising from the supra- orbital prominence, rapidly approximate as they proceed backward, meeting each other on the median line, uniting, and running backward as the frontal portion of the sagittal crest. The part of the frontal forming the temporal fosse is irregularly concave ; the orbital part is smooth, plane, and vertical; the two are separated by a slight ridge running from the supra- orbital protuberance to the sphenoid. The interior of the frontal is perfectly smooth for its cerebral portion, the surface being only marked with two slight depressions for the rhinencephalon; the walls of the nasal chamber, on the other hand, are irregular, and marked in various ways. A strong ridge de- pends from the median line, marking the junction of the vertical ethmoidal plate, and dividing the interior into lateral moieties. The superior portion of the surface of each is hol- lowed for the reception of the upper turbinal convolutions ; the lower portion (inner surface of orbital plate of frontal) is marked by several (four or five) longitudinal, horizontal lines, nearly parallel with each other, corresponding with the middle convolutions of the turbinate bones. There are a few other irregularities, not necessary to particularize, which, like those just described, relate to the convolutions of the spongy bones. The nasals remain distinct from each other, and from the surrounding bones with which they are suturally connected. They are long and narrow, nearly straight, in mutual apposition along the median line. The anterior half is narrowly linear, the posterior has a lateral expansion; so that the fronto- and maxillo-nasal sutures enclose a diamond- shaped space upon the lower forehead ; the naso-intermaxillary takes the place of the naso- maxillary suture for the anterior third of the bone. These sutures are squamous, the nasal overlapping the maxillary and intermaxillary. The superior surface of the nasals is smooth and nearly plane ; the inferior aspect of the conjoined bones is a half-cylinder. The nasals OF DIDELPHYS VIRGINIANA. 59 terminate in a point, which falls short of the end of the jaw; “rostral’’ bones are not developed. : The ascending ramus of the itermaaillary is wedged in between the nasals and maxillaries, extending about an inch upon the cheek, overriding the side of the nasal, and being in turn overlapped by the maxillary. The rami are curved, with the convexity outward, at first divaricating from each other to increase the calibre of the nasal aper- ture, and then somewhat approximating, and also each curling upon itself, as the alveolar borders come to meet on the median line in front. The palatal plate of each bone is almost divided into two longitudinal parts by an extensive fissure — the incisive foramen — which nearly reaches the alveolus. This foramen is not completed behind by the intermax- illary ; it is simply a slit, so far as that bone is concerned. The median moiety of the palatal plate is an extremely long and attenuated spicula whose extent is not visible from below in the articulated skull, as the palatal plate of the maxillary overlaps it. The lateral moiety is much shorter, as well as broader; it terminates at the anterior border of the socket of the upper canine. The intermaxillaries are never confluent with each other or with surrounding bones. Each contains five teeth. The mawillary is large, and has numerous strong processes. Its surface is smooth; the foramen appears just above the first or second premolar, opening upon a slightly depressed space, in advance of which is the bulging that denotes the insertion of the great canine. The nasal plate or process curves toward the nasal bone, and unites with the latter for about half its length, being anteriorly cut off by the intermaxillary ; the poste- rior prolongation of this plate, in the form of a sharp pointed process, reaches the frontal. A deep excavation in the border of the bone, just below the process, indicates the seat of the lachrymal. Posteriorly the bone sends upward a large, stout, zygomatic process, for union with the malar, in a manner to be more particularly noticed under head of the latter. The alveolar border is strongly developed, particularly behind; it narrows anteriorly, and is deficient at the wide interspace between the first and second premolars. The palatal plate or process meets its fellow along a perfectly straight median line. Besides being nicked an- teriorly to perfect the incisive foramen, it is deficient posteriorly, where the palatal plate of the palatine completes the bony roof of the mouth; and is, moreover, deeply fissured to form the posterior elongated vacuity, described below. Both upper and under surfaces of the palatal plate are nearly smooth ; on the latter, however, a shallow but distinct groove may be observed running from the posterior fissure to the root of the canine. The posterior bor- der of the bone, between the malar and palatine, is thick, smooth and concave ; it is con- tinued as a process just behind the last molar, taking part in the formation of the most postero-external of the ten openings in the bony palate. The share that the zygomatic process takes in the formation of the outer wall of the orbit is inconsiderable, though greater than appears on viewing the bone from the outside. The floor of the orbit — what little there is of it—is maxillar. The surface of this part of the bone is traversed by a groove, in which lies the maxillary division of the trigeminus, which is conducted by this means along the orbital floor into a bony canal, whose outlet is the maxillary foramen. The bone bears upon the inner surface of its ascending plate, the lower or anterior turbinal bone —a spongy convoluted mass, of extremely delicate texture, which nearly fills that part of the nasal chamber not occupied by the ethmo-turbinal. The maxillary is permanently distinct from all of its surroundings. The alveolus bears eight teeth 60 THE OSTEOLOGY AND MYOLOGY The malar, which is a large, stout bone, forms the greater part of the zygoma. In general form it is a little arched, a little curved outward, compressed, expanded vertically, and with extensive, very oblique sutural terminations. The anterior extremity chiefly unites with the maxilla by overlapping; but its extreme point is received into a slight pit in the latter bone, while the extremity of the zygomatic process of the maxilla is firmly wedged in a similar but larger depression in the malar itself. The suture is thus made remarkably strong. The posterior extremity of the bone is more irregular.