136 ANNALS NEW YORK ACADEMY OF SCIENCES 



(c) Capiti-mandibularis profundus. — This deep slip must have taken 

 its origin on the inner side of the temporal mass and have been inserted 

 on the ascending process of the dentary. 



Pterygoideus anterior. — To counterbalance the pull of the capiti-man- 

 dibularis mass the anterior pterygoids must have played an important part. 

 The ascending process of the pterygoid is quite large, thus giving an at- 

 tachment for large pterygoids. There is no need for the complex mus- 

 culature of the type found in streptostylic reptiles, so it was probably 

 more of the chelonian or rhynchocephalian type. The pterygoid bones 

 of Cynognathus are bowed inward to the mid line, covering the primary 

 skull base and forming on each side an extended longitudinal fossa for 

 the origin of the anterior pterygoid muscle. Although the articular, pre- 

 articular and angular bones are small, they seem sufficiently large for the 

 insertion of the anterior pterygoid in the normal place at the back part 

 of the jaw. The insertion of this muscle in all recent reptiles is at or 

 near the rear end of the jaw, and in view of the general fact that inser- 

 tion areas are less variable than origin areas, there seems no reason for 

 regarding Cynognathus as having an exceptional insertion of the anterior 

 pterygoid. The pull of the anterior pterygoid muscle would counter- 

 balance that of the capiti-mandibularis mass and take some of the strain 

 off the weak quadrate articulation. The dentary, imbedded in the tem- 

 poral mass, would thus act as a cushion and the action of the jaw would 

 be the same as though a strong articulation were present. That this 

 action of the jaw was effective is shown by the robust character of the 

 teeth and jaw. If Cynognathus had relied only upon the old articulation 

 by way of the quadrate and articular, a bite on a piece of bone might have 

 dislocated the jaw. As the dentition is carnivorous, the jaws did not 

 have much lateral motion. The reason Cynognathus had a large ascend- 

 ing ramus of the dentary is that it was derived from therocephalians and 

 ultimately from primitive Theromorpha, in which the coronoid bone was 

 functionally replaced by an obliquely ascending process of the dentary. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Depressor mandibular. — This muscle was in its usual position, as the 

 posterior end of the mandible shows the insertion. In all reptiles there 

 is a remnant of the C 2 md of Ruge, and from this the mammalian digas- 

 tric was probably derived. With the loss of the posterior end of the rep- 

 tilian jaw the depressor mandibulse of these forms would disappear and 

 a new digastric would be formed from this sheet of muscle that has per- 

 sisted from the fish stage. The long ventral muscles of the throat could 



