THE OSTEOLOGY OF THE DIADECTIDAE 155 



statement made by Cope that the last order was ancestral to the 

 turtles. It is not assumed that the Diadectidae are the direct ances- 

 tors of the turtles, nor can this statement be made in regard to any 

 of the Chelydosauria; for in none is there any beginning of the 

 development of a plastron, and only an incipient carapace, which 

 cannot" be regarded as determinative, as it also occurs in an amphibian 

 Dissorophus, of the same beds. The only features of the body skele- 

 ton prophetic of the turtles are the beginning of the carapace and 

 the number of presacral vertebrae, which is eighteen — a number 

 which also occurs in many of the Cotylosauria. It is in the skull 

 that the testudinate affinities appear. For convenience of discussion, 

 the seven most important points are listed below: 



1. The form and relations of the quadrate. 



2. The degenerate palate and the disappearing transverse bone. 



3. The absence of teeth on the pterygoids and palatines. 



4. The absence of a parasphenoid process on the basisphenoid 

 bone. 



5. The absence of prevomers and the presence of an anteriorly 

 placed single vomer (parasphenoid). 



6. The method of entrance of the internal carotid arteries into 

 the skull. 



7. The presence of paired descending plates from the skull roof 

 anterior to the brain cavity. 



These will be discussed in order. 



1. The form and relations of the quadrate. In the order 

 Cotylosauria there are, as in the Stegocephalia, but five openings 

 in the skull roof — the nostrils, the orbits, and the pineal foramen. 

 The quadrate region is covered by dermal bones, so that the 

 quadrate bone is seen only from behind or below to any extent. 

 In the Chelydosauria, as described above, the quadrate appears 

 on the side of the skull, and forms a portion of the side wall, 

 surrounding an opening, the meatus auditus externus; the meati of 

 the two sides forming a third pair of openings to the interior of the 

 skull. As pointed out in the description, the quadrate bears exactly 

 the relations to the bones of the roof and lower portion of the skull 

 that the same bone bears in the turtles, and there is a suggestion of 

 an overhanging hook on the posterior edge, indicating the beginning 



