524 Mr. D. M. S. Watson on the Gynodontia. 



Cynognathus agrees very closely with Gomphognathus in 

 structure. 



It is advanced over Thrinaxodon in : — 



1. Further retraction of the anterior border of the nasal. 



2. Loss of the facial exposure of the septomaxilla. 



3. Further specialization of the dentition. 



4. Further reduction of the hinder end of the lower jaw. 



5. Complete loss of the quadrate ramus of the pterygoid. 



6. Regression of the anterior margin of the pterygoid and 



loss of the process separating the palatine and vomer. 



7. Backward extension ot the vomer. 



Protacmon, the most advanced Cynodont known, differs 

 from its ally Goniphognathus in the complete loss of the 

 quadrate ramus of the epipterygoid, thus freeing the quadrate, 

 and in a further reduction in size of the bones of the back of 

 the jaw. 



This loss of any connection between the epipterygoid and 

 the quadrate is the logical completion of the whole evolution 

 of this region in Theriodontia. 



In early Gorgonopsids there is a normal quadrate ramus of 

 the pterygoid, passing behind the pterygoid ramus of the 

 quadrate. 



In Arctognathus the pterygoid fails to reach the quadrate 

 by a few millimetres. 



In Thrinaxodon the pterygoid has a considerable quadrate 

 ramus lying below that of the epipterygoid, which itself 

 passes back to the quadrate. 



In Gomphognathus poly phagus there is no quadrate ramus 

 of the pterygoid, but the hinder surface of the quadrate ramus 

 of the epipterygoid is supported by a long splint from tlie 

 quadrate. In G. broioni this quadrate splint is lost and the 

 epipterygoid and quadrate only just meet. In Protacmon 

 they are widely separated and the quadrate is supported 

 solely by the squamosal. 



Thus, known Cynodonts give us a series of morphological 

 stages, which bridge over the gap between the advanced 

 Gorgonopsids like Arctognathus and so remarkably mammal- 

 like an animal as Protacmon. 



I am indebted to the Percy Sladen Fund for assistance in 

 visiting South Africa and there collecting the type-skull of 

 Protacmon. 



