60 GUIDE TO EEPTILES AND AMPHIBIANS. 



curved in a swan-like fashion ; but from the fact that the vertebras 

 of the neck articulate with one another by means of slightly concave 

 surfaces (instead of by ball-and-socket joints), such a curvature was 

 apparently impossible. 



OF UNCERTAIN POSITION. 



Group PLACODONTIA {extinct). 

 (Case 5.) 

 In this place may be mentioned the extinct Triassic reptiles 

 known as Placodus and Cyamodus (51), mainly represented by their 

 skulls. These skulls are characterised by their broad and flattened 

 shape, and by the presence on the palate of a small number of bean- 

 like teeth, evidently adapted for crushing hard substances ; in 

 addition to which there are two or three pairs of chisel-like teeth 

 in the front of the jaws. The systematic position of these reptiles 

 is still a matter of uncertainty. The cast of a fine skull of Cyamodus 

 is exhibited. 



Order X.— THEROMORPHA (Mammal-like Reptiles— extinct). 



(Case 5.) 

 The members of this extinct group are confined to the Permian 

 and Triassic epochs, and are abundant in South Africa and Russia. 

 They are connected on the one hand with the Stegosaurian Amphibia, 

 and on the other with the Monotreme Mammalia, to the latter of 

 which they exhibit resemblances in the structure of the skeleton, 

 and of which they seem to have been the ancestors. In the skull 

 the quadrate is fixed, and there is a large parietal foramen ; the 

 pubis and ischium of each side of the pelvis meet in the middle line 

 to form a symphysis ; the shoulder-girdle consists of three bones, 

 and the humerus has a perforation (entepicondylar) at the lower 

 end. The two temporal arches of the skull have coalesced into one, 

 corresponding to the cheek-arch of Mammals. The group is divided 

 into the following sub-orders : — ■ 



I. Pariasauria. — The skull is completely roofed over by Sculp- 

 tured bones, so that the only vacuities on the upper surface 

 are formed by the nostrils, eye-sockets, and parietal foramen . 

 The teeth are relatively small, and form an even series. 

 Pariasaurus (52) was a large uncouth reptile, measuring 

 nearly 8 feet in length (inclusive of the short tail) and 

 between 2 and 3 feet in height. 



