Explanation of Plate XIII 



Restoration of the jaw muscles of Tyrawnosaurm, representing the Diuosauria 

 Theropoda, and of Cynognathus, representing the Therapsida 



Fig. 1. — Tyrannosaurus. Superficial muscles. 



As in many other reptiles, the capiti-mandibularis (C m, s.) was probably 

 differentiated into three layers, the more superficial layer of which (Cm.) is 

 analogous with the masseter of higher types of mammals. The supratemporal 

 fenestra was probably filled by tbe middle and deep layers. The pterygoideus 

 anterior may have filled the space beneath the preorbital fenestra. 



Figs. 2-5. — Cynognathus. Jaw muscles. 



A distant approach to mammalian conditions is seen in the reduction of the 

 posterior jaw elements and in the enlargement of the ascending ramus of the 

 dentary. Very probably the outer layer of the capiti-mandibularis was attached 

 to the lower border of the zygomatic arch in the position of its homologue, the 

 mammalian masseter. The middle and deep layers fill the temporal fossa as 

 in mammals. Cynognathus probably still retained the reptilian pterygoideus 

 anterior which does not appear to be homologous with either of the pterygoid 

 muscles of mammals, but may possibly be represented by the tensor tympani. 

 The capiti-mandibularis was in a better position to give rise to these muscles. 

 A true depressor mandibular (D. m.) was doubtless present and the general ar- 

 rangement of the muscles was reptilian in type. 



A. = articular (inside) D. = dentary 



Ang. = angular P. art. == prearticular 



Ait. = articular (outside^ Sur. a.= surangular 



Cr. = coronoid 



