ADAMS, PHYLOGENY OF THE JAW MUSCLES 147 



and is inserted on the coronoid region of the jaw along with the rest of 

 the mass. (b) The medius slip also is not separate, the fibers being 

 under the superficial slip and extending to the parietal region, (c) The 

 profundus is represented by the deep part of the mass, (d) The cranio- 

 pterygoid muscles are the special muscles developed in the pterygoid re- 

 gion of the streptostylic reptiles and are new developments. 



Pterygoideus anterior and pterygoideus posterior. — The so-called ptery- 

 goid muscles of reptiles include two quite different groups; the posterior 

 group represents the deepest part of the capiti-mandibularis mass. It is 

 usually named the pterygoideus externus or external pterygoid, but it is 

 probably only partly homologous with the mammalian external pterygoid. 

 I have named it the capiti-mandibularis profundus. It is often sub- 

 divided into two heads, which may be named caput 1 and caput 2. The 

 anterior pterygoid called by Mivart "Internal pterygoid" and in this 

 paper "Pter} T goideus anterior'' represents the anterior part of the ad- 

 ductor mass or CxSmd of fishes and is foreshadowed in Polypterus. It 

 may be the homolog also of the so-called "temporalis" of urodeles. It is 

 far in front of the "Pterygoideus externus" and runs in the opposite 

 direction, crossing it at a wide angle on its way from the orbital region 

 to the back of the mandible. It is always present in the reptiles and acts 

 in concert with the capiti-mandibularis mass in closing the jaw. More 

 in detail the action is described in the section on reptiles. 



MUSCLES OF THE DEPRESSOR GROUP 

 (INNERVATED BY VII) 



The depressor mandibular acts as the depressor of the mandible. It is 

 constant throughout the Sauropsida. It was retained throughout the 

 series until the mammal was evolved, when probably a new slip was sepa- 

 rated from the mother mass and the new depressor of the mammal was 

 formed. 



Futamura (1907, p. 570) on comparative and embryological evidence 

 stated that the stapedial muscle of the mammals and man comes from 

 the proximal or upper part of the muscles of the hyoid arch (innervated 

 by VII) — that is, the mother mass of the depressor mandibular — and that 

 the reptiles also derive their stapedial muscles from the same source. 

 Futamura says : 



Der M. stapedius stammt von den Muskelfasern die sich vom proximalen 

 Teil der Hyoidmuskulatur nach Gehorknochelchen begeben : Bei den Reptilien 

 (Krokodil, Lacerta) ist er mehrfach und in seine Funktion auch verschieden 

 von der der Vogel und Saugethiere. Nach Killian finden sich beim Krokodile 

 Ohrmuskeln, die Heber, Herabzieher der Ohrklappe und Spanner des Trommel- 



