490 DR. R. BROOM ON THE STRUCTURE [-^W- ^^j 



5. On the Structure o£ the Theriodont Mandible^ and on its 

 Mode of Articulation with the Skull. By B,. Broom, 

 M.D., B.Sc, C.M.Z.S., Victoria College, Stellenbosch, 

 S. Africa. 



[Received March 25, 1904.] 

 (Plate XXXY.* and Text-figui-e 100.) 



As there are very strong reasons for believing that all mammals 

 are descended from a Theriodont ancestor, all new light on the 

 structure of the Theriodonts must be welcomed by morphologists. 



Only a very limited number of good Theriodont skulls have 

 been discovered, and these have almost all been already described 

 by Prof. Seeley. The majority of the tyj^es are in London, where 

 they are readily accessible to European and American students, 

 but in the museum at Grahamstown, S. Africa, there are two or 

 three important specimens that seem worthy of more minute 

 study than has yet been given to them. 



Recently, through the kindness of Dr. Schonland, I made a 

 careful study of the Grahamstown specimens, with a. view to 

 getting more definite information about the structure of the lower 

 jaw and its mode of articulation. 



Perhaps the most valuable specimen in the collection is the 

 skull wliich forms the type of Cynognathus platycejis. It is an 

 almost perfect skull, slightly crushed, and wanting only the snout. 

 The mandibles are in position and the matrix has been beautifully 

 cleared away. The specimen has been figured by Prof. Seeley 

 and described at considerable length (1). The figure of the under 

 surface unfortunately has been executed in such a manner that 

 the student cannot readily determine what is matrix and what is 

 bone, while there are no letters given to indicate the position of 

 the dififerent elements. 



The mandible is formed mainly by the dentary bone, which is 

 so largely developed that when the jaw is viewed from the outer 

 side all the other elements are completely hidden except in the 

 articular region. 



The dentary has a very large coronoid process, which extends 

 almost as far back as the plane of the articular region. The 

 fractured anterior part of the bone shows the small splenial lying 

 in a shallow groove on the inner side of the bone. Posteriorly 

 the dentary received on its inner side the angular and articular 

 bones. The surangular lies on the inner side of the bone near 

 the base of the coronoid process. The specimen does not show 

 very satisfactorily the sti-ucture of the posterior part of the 

 dentary. 



The articular is fairly well seen in the specimen. It forms a 

 broad articulation with the quadrate and possibly with a small 



* For explanation of the Plate, see p. 498. 



