SKELETON OF MONOTREMATA . 



321 



middle of the smooth alveolar border, the other close to the end 

 of the ramus. In no mammiferous ammal does the lower jaw 

 bear so small a proportion to the skull or to the rest of the 

 skeleton as in the Echidna. 



In the Ornithorhynchus the lower jaw, fig. 204, E, is much 

 more developed. Each ramus commences posteriorly by a large 

 convex condyle. The ascending ramus is nearly horizontal, 

 flattened below, and continued upward in the form of a low 

 vertical compressed plate, on each side of which there is a deep 

 fossa. The ascending is continued by a gentle curve into the 

 horizontal ramus, and the angle of the jaw is very feebly in- 

 dicated. The horizontal ramus suddenly expands and sends off 

 above in the same transverse line two short obtuse processes, both 

 of which might be termed ^ coronoid ; ' this structure is peculiar 

 to the Ornithorhynchus. The innermost process, c, although 

 the largest, is the superadded structure, as it affords insertion to 

 the internal pterygoid. The socket, d, e, for the horny grinder, 

 is shalloAv ; its floor is perforated by several large foramina. The 

 dental canal divides ; one branch opens by a wide elliptical fora- 

 men on the outside of the ramus immediately anterior to the 

 alveolus, the other terminates at the lower part of the end of the 

 ramus. The rami of the jaw converge and are united at a sym- 

 physis of more than half an inch in length ; there they become 

 expanded and flattened, then again disunite, and are continued 

 forward as two spatulate processes, b, which diverge from each 

 other to their broad rounded terminations, 

 and are situated just behind the inflected 

 extremities of the similarly separated pre- 

 maxillaries, ib. A, and fig. 205, 22. On the 

 outer sides of the upper surface of the 

 broad symphysis are the long and narrow 

 sockets of the two anterior trenchant horny 

 teeth. The Monotremes diflfer from the 

 Marsupials in the absence of the inflected 

 process developed from the angle of the 

 lower jaw. 



The exoccipitals, fig. 205, 2, h, and 

 superoccipital, ib. 3, are separate in the 

 skull of the young Ornithorhynchus liere 

 figured of the natural size. The mastoid, 

 ib. e, e, contribvites to part of the occi- 

 pital surface, and advances anteriorly to the small cranial expan- 

 sion of the squamosal at /. This expansion does not exceed 



VOL. II. Y 



205 



Skull of young Ornithorliynchus, 

 iiat. size. 



