OF DIDELPHYS VIRGIXIANA. 6B 



readily comes apart without maceration. The rami meet at an acnte angle (of about 30'') ; 

 the symphysis is an inch long, ver}^ narrow, and almost horizontal ; the course of each 

 ramus is exflected, so that the two sides lie nearly parallel for this distance. The body of 

 the bone is twisted upon itself, so that the alveoli, at first perpendicular, lean more and 

 more outward from behind forward ; the teeth, as a series, thus lie oblique to the long axis 

 of the jaw ; and while the molars are upright, the premolars slant a little outward, and the 

 canine and incisors have a successively increasing outward and forward obliquity, {vide figs. 

 7 and 8). There are twelve teeth on each side. Taking the line of the alveolar border as 

 the middle longitudinal axis of the mandil)le, the lower border of the bone represents an 

 unbroken arc of a circle, from incisor teeth to condyle: the latter is situated much above 

 the long axis just assumed. 



There is no vestige of ''an angle of the jaw," as such. The portion of bone that forms 

 this part in placental carnivores is here inflected as it Avere, and appears as a sharp ridge, or 

 plate, extending at right angles with the plane of the coronoid plate into the inter-ranuil 

 space toward its fellow of the opposite side. This ridge ends in a sharp pointed process, 

 directed backward. The lower border of the jaw presents no noteworthy features until we 

 reach the beginning of this remarkalde process ; there it becomes flattened, and transversely 

 widened ; a smooth, flat triangular surface being produced, mostly by the process just 

 described, but partly, also, by a ridge of bone looking horizontally outward, and continued 

 directly up to the condyle. The inner pointed process is separated from the corresponding 

 side of the condyle by a deep and wide notch. A small and rather irregular groove leads 

 obliquely from the notch to the outside of the condyle. The articular surface of the latter 

 is greatly extended in its transverse diameter, and very narrow in the opposite direction. 

 The coronoid process is enormously developed, seeming to virtually constitute the " ascend- 

 ino- ramus of the jaw." In the articulated skull, it reaches above the superior margin of 

 the zygoma. Its shape is liable to considerable variation witli individuals ; besides which, 

 its general direction from above downward appears to change with age. In a young animal, 

 I find that the backward obliquity is sufficient to carry the apex directly over the con- 

 dyle ; in adult specimens the coronoid is much more upright, as represented in the cut. 

 The coronoid is very thin — almost diaphanous ; the anterior border is thickened, and 

 descends with a slight forward convexity to the main part of the jaw a little behind the last 

 molar ; the apical and posterior borders are very sharp ; the former is obtusely rounded, the 

 latter descends in a nearly straight (but sometimes sinuous or otherwise irregular) line a 

 little below the level of the condyle, up to which it then curves, forming a well-marked 

 notch. The inner surface of the coronoid is smooth, and continuous with the general sur- 

 face of the jaw ; the outer is roughened for muscular attachment, and forms a broad, shallow 

 fossa, determined by the thickening of the bone along the anterior border of the coronoid 

 itself, and the external ridge along the under border of jaw, leading up to the condyle. 

 One Avould scarcely predicate, from the general contour of the mandible, the great force 

 that the animal can exert with its teeth ; but the enormous temporal and masseter muscles, 

 acting upon the greatly expanded coronoid process, more than compensate for the disadvan- 

 tages that result from the length and straightness of tlie jaw, and the position of the con- 

 dyle. The dental nerve and vessels enter the jaw at a large oval foramen situated at the 

 middle of the base of the internal angular process above described. A similar but smaller 



