ADAMS, PHYLOGENY OF THE JAW MUSCLES 93 



palate and into the maxillffi. This muscle has an important part in the 

 closing of the long mandibles. 



The jaw muscles of the alligator are of the type found in monimostylic 

 reptiles. The fixed pterygoids and palatines demand no muscles for their 

 movement and consequently the muscles that function in Varanus and 

 other streptostylic forms are not differentiated. The muscles of the 

 capiti-mandibularis are pinched up, especially in the temporal section, as 

 the temporal fossa is much reduced in most modern Crocodilia. This 

 reduction is secondary, for the early fossil forms show no such reduction, 

 but rather the opposite condition, with large supratemporal and latero- 

 temporal fenestrse and small orbits. Most of the Mesosuchia show a 

 large supratemporal fenestra as in Teleosmirus, where the fenestrse are 

 of large size. There is a progressive closing of this fenestra from Teleo- 

 sauriis of the Jura through Geosaurus, Goniopholis and Tomistoma. 

 This closing and the pinching in of the temporal region means that the 

 outer, or masseter, slip of the capiti-mandibularis (capiti-mandibularis 

 superficialis) is enlarging and is taking over the principal work of closing 

 the jaws, while the temporal slip is being progressively reduced. The 

 pterygoid muscle is peculiar in that its anterior portion (pterygoideus 

 anterior) has dug its way under the eye and into the maxilla, extending 

 to a point far anterior to the eye, since it lies on the palatine and pene- 

 trates far into the maxilla. The depressing of the jaw is done by the 

 usual muscle, the depressor mandibula?. The Crocodilia spend much of 

 their time lying on the mud, and the action of the depressor in this posi- 

 tion is to raise the head if the mandible is resting on something fairly 

 firm. The Crocodilia have some peculiar external ear muscles, necessary 

 in the crocodile from the advanced condition of the ear with its external 

 flap. The nerves divide the muscles into two groups. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY Y.J 



Capiti-mandibularis. 



(a) Capiti-mandibularis superficialis. 



^ Masseter portion. 

 (/;) Capiti-mandibularis medius. 



= Temporal slip. 

 (c) Capiti-mandibularis profundus. 



= Deep part of temporal mass. 

 Pterygoideus anterior. 



= Pterygo-mandibularis, Bradley. 

 Pterygoideus posterior, Lubosch. 



