118 A REVISION OF THE COTYLOSAURIA OF NORTH AMERICA 



of the basisphenoid, but was still of good size. The vertebrae were notochordal, 

 the neural arches were low and broad, with the zygapophysial faces horizontal and 

 with short stout spines. This character of the neural arches of the Cotylosauria 

 seems to be one of the most persistent features and to be peculiar to the primitive 

 reptiles. (It occurs in the Proganosaurian Mesosaurus, which is very probably an 

 aquatic adaptation of the Cotylosaurian type and retains many of the primitive 

 characters.) The ribs were single-headed and attached to the transverse processes. 

 (It is asserted by Williston and Moody that the ribs of Eosauravus are intercentral 

 in position. This is the most puzzling thing about the specimen; if it were not for 

 this, it would form a very satisfactory connecting link between the reptiles and 

 amphibians as far as the posterior portion of the skeleton goes.) There were about 

 twenty-three or twenty-four presacral vertebrae, two sacrals, and twenty or more 

 caudals. Abdominal ribs were present. The shoulder girdle of the primitive form 

 had the coracoid and procoracoid united with the scapula; cleithrum present. The 

 ilium and ischium were flat and plate-like and the bones of the opposite sides met 

 in a straight symphysis, but were not united. The articular surfaces of the limb 

 bones were well formed. The two ends of the humerus were turned almost at a 

 right angle to each other and there was an entepicondylar foramen. The carpus 

 had radiale, intermedium, and ulnare, probably two centrale, and five bones in the 

 distal row. The phalangeal formula was almost certainly 2, 3, 4, 5, 3. The tarsus 

 had tibale, fibulare, five bones in the distal row, and a pisiform. 

 The departure from this primitive type was: 



1. The change in position of the bones over the temporal region. In some the 



supraoccipital plates became vertical, the quadratojugal took a position 

 on the posterior surface of the quadrate, and the temporal region was 

 covered by only two bones. In others the temporal region retained the 

 three bones of the primitive cover. 



2. The development of an epiotic notch and the final uncovering of the quadrate 



in some forms. 



3. The separation of coracoid, procoracoid, and scapula. 



4. The ischium and pubis became more nearly vertical and met at the symphysis 



in an angle. 



5. The tarsal bones of the proximal row became united in a single large element. 



6. The phalangeal formula became reduced in some. 



There is no single one of the Cotylosauria that can be considered as an ancestral 

 form of the other reptiles. It is impossible to derive the Diapsidan and Synapsidan 

 types from the known Cotylosaurs; perhaps some primitive form may have given 

 rise to the single and double arched types by perforation of the roof, but the result 

 of these studies has been to render this less probable and certainly to exclude any 

 form yet discovered from such an ancestral position. The theory which places 

 Sphenodon as the representative of the primitive form derived directly from the 

 Cotylosauria is inadequate to explain the facts. 



