OF DIDELPIIYS VIRGINIANA. 65 



cranial depth is due to the elevated comb of bone. The intcr-mastoid distance is less than 

 the o-rcatest inter-alveolar. The bony palate occupies about three-fifths of the length of 

 the floor of the skull. The circumference of the constricted part of the cranium scarcely 

 exceeds that of the muzzle measured just in front of the premolars. These various ad- 

 measurements display, very instructively, the predominance of the sensory over the ratioci- 

 native faculties. The swollen and elongated face contrasts strongly with the contracted 

 brain case. The expanded zygomata and enormous temporal fossa?, bounded by high, sharp 

 crests, arc as suggestive as the massive jaws themselves, with their long tushes. The shape 

 of the bare skull betrays the cunning and ferocity of the beast, as plainly as these qualities 

 are expressed by its physiognomy during life. 



The base of the skull is semicircular in contour, continuously bounded laterally and 

 above by the strongly developed occipital (no longer " lambdoidal") crest, the backward 

 projection of which converts what would otherwise be a plane into a depression, all of which 

 is roughened for the firm implantation of the nuchal muscles. This space is always trav- 

 ersed by the very irregular curved line, marking the junction of the par- and ex- with the 

 super-occipitals and mastoid : and, except, perhaps, in very old skulls, by a second line, 

 straight, vertical and median, at the junction of the exoccipitals over the foramen magnum. 

 The latter is broadly oval in shape, widest transversely ; its lower border is on a line witli 

 the straio-ht edge of the base of the skull. The condylar protuberance is considerable ; the 

 condyles are lateral in position, looking outward, forward, and downward ; superiorly they 

 are separated by a wide interval, along which the rim of the foramen is thin and sharp ; in- 

 feriorly the articular surfaces nearly touch each other. A deep notch separates them from 

 the paroccipitals : the latter are stout, conical processes, projecting straight downward, till 

 their apices reach the plane of the posterior molars. Their bases abut externally against 

 the mastoid processes — two roughened, irregular prominences that complete the lateral cor- 

 ners of the semicircle. A small foramen perforates the posterior aspect of the mastoid, high 

 up, opposite the most posterior root of the zygomatic portion of the squamosal. The con- 

 dylar canals emerge from under the edge of the articular facets, in the depression between 

 the latter and the paroccipitals. A minute foramen perforates the basioccipital on either 

 side a little in advance of the condyles. 



The superior aspect of the skull is abruptly bounded posteriorly by the transverse ele- 

 vated occipital crests ; laterally by the strong, wide-spread, curved and arclied zygomata 

 and perpendicular surface of maxillary and premaxillary bones. The sides of tlie cranium 

 proper are depressed, roughened for the attachment of the temporalis and masseter, and 

 present the hour-glass contraction already mentioned. They are only expanded and 

 vaulted posteriorly between the roots of the zygomata. The surface is traversed by lines 

 denoting the fronto-, spheno-, squamo- and occipito-parietal sutures ; the latter is very fre- 

 quently obliterated with age. The sides of the brain-case meet above, at an angle along 

 the median line, where one of the distinguishing peculiarities of the opossum's skull is 

 exhibited. This is the development of an enormous " sagittal " or interparietal crest or 

 comb, extending unbroken from the crista occipitalis to the middle of the os frontis. The 

 elevation of this crest increases with age, as usual, and is moreover subject to variation with 

 individuals ; beside which discrepancies, I think that I observe a difference dependent upon 

 sex ; the crest in the male being higher and thicker. The lateral moieties of the parietal 

 portion of the crest usually remain separable, except in very old skulls. Owing to the 



MKMOIllS B03T. SOO. NAT. HIST. VOL. II. 17 



