MAMMAL-LIKE EEPTILES. 



61 



Pig. 61. 



II, COTYLOSAURIA. — Typically a North American group, distin- 

 guished by the roofing-over of the temporal region of the 

 skull (sometimes with a small foramen), the presence of 

 more than 2, 3, 3, 4, 3 joints to the toes (the number in the 

 Pariasauria). Procolophon (59) and Empedias (58) are well- 

 known genera, in which the cheek-teeth have transversely 

 elongated molar-like crowns. 



III. Theriodontia. — The temporal region of the skull shows 

 large vacuities, and the single temporal (zygomatic) arch 

 in some cases {Cynognathus, 54) exhibits a vacuity indicative 

 of its double origin. The teeth are typically differentiated 

 into incisors, tusks, and a cheek-series ; the lower tusks 

 biting in front of the upper pair. Gcdesaurus (57) and 

 Cynognatltvs (54) are typical forms. The position of Trity- 

 lodon (56), in which the teeth are of a different type, and those 

 of the cheek-series extremely Mammal-like, is uncertain ; the 

 skull has the pre-frontal and post-frontal bones of Eeptiles. 

 IV. DiCYNODONTiA. — In this group the teeth are reduced to a pair 

 of long per- 

 man e n tl y - 

 growing upper 

 tusks, or are 

 altoge t h er 

 wanting ; and 

 the jaws were 

 probably 

 sheathed in 

 horn. The 

 quadrate - bone 

 is greatly 

 elongated, and 

 thus forms a 

 pedicle for the 

 support of the 

 lower jaw. 

 Dicy n don 



(63), Udeiwdon, and Ptyclwsiagum, are well-known examples. 

 Casts of skulls and other parts of the skeleton of several of the 



more striking forms, such as the theriodonts CgnognatJ/us (54) and 



JElurosaurus (53, fig. 61), as well as Dicgnodoii (63) and Paria- 



saurus (52), are exhibited. 



Right side of Skull of a Theriodont {JElurosaurus 

 felinus), two-thirds nat. size, with two upper 

 teeth nat. size (a, 5), from the Triassic Forma- 

 tion, Cape Colony. Behind the large socket 

 of the eye the skull is broken away. (No. 53.) 



