MAMMAL-LIKE KEPTILES. 



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). Procotophon (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. Galesaurus (57) and 

 Cynognathus (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 Reptiles. 



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



of long per- 



-i Fig. 61. 



man e n 1 1 y - 



growing upper 



tusks, or are 



altogether 



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. 



J) icy n o don 



(63), Udenodon, and Ptychosiagnm, are well-known examples. 



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



more striking forms, such as the theriodonts Cynognathus (54) and 



JElurosaurus (53, fig- fil), as well as Dicgnodon (63) and Paria- 



saurus (52), are exhibited. 



Right side of Skull of a Theriodont (^lurosaurus 

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

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

 tion, Cape Colony. Behind the large socket 

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



