156 E. C. CASE 



of a closure of the bone to form a complete tympanic ring. But we 

 know that there are many turtles in which the tympanic is open 

 behind. Baur gives the following list: Amphichelydia, Dermate- 

 mydidae, Staurotypidae, Kinosternidae, Toxochelydae, Platyster- 

 nidae, Emydidae, and Adelochelys. 



2. The degenerate palate and the disappearing transverse. It 

 is evident that the palate is very different from that of the Cotylo- 

 sauria and from any of the more primitive orders of reptiles, as 

 Proterosauria, Proganosauria, Rhyncocephalia, etc. There is no 

 anterior rostrum (presphenoid auct.) on the basisphenoid; there are 

 no paired prevomers; the palatines and transverse are degenerate; 

 there is no descending buttress on the external process of the ptery- 

 goids for the lower jaw. The condition of the anterior rostrum of 

 the basisphenoid and the prevomers is discussed below. It is appar- 

 ent that the palate is in process of change toward a new type. It 

 may well be that the Diadectidae show an extremely degenerate exam- 

 ple of this change, and that the more successful line showed no such 

 violent differences from the parent form at any stage. The ridge-like 

 form of the palatines on the edge of the maxillaries speak of their 

 extension toward the middle line to meet a median element extend- 

 ing far posteriorly — an element which already exists, but from which 

 they are separated by the long antero-posterior vacuities of the 

 palate. The firm attachment of the pterygoids to the maxillaries is 

 indicative of the final disappearance of the transverse, one of the 

 most characteristic features of the Chelonian palate. 



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

 all other forms of the Cotylosauria there are teeth on the pterygoids 

 or palatines, or both. Two specimens show that they are absent in 

 the Diadectidae. 



4. The absence of a parasphenoid process on the basisphenoid 

 bone. If we accept without argument or review the position taken 

 by Broom, that the true vomer is the parasphenoid in a new position 

 and with a new function, and that the paired elements of the reptilian 

 skull usually called vomers, are really distinct elements, then the 

 identification of the parasphenoid in the skull becomes of extreme 

 morphological importance. It has been shown by Howse and 

 Swinnerton (Howse and Swinnerton, 1901) and Siebenrock (Sieben- 



