ADAMS, PHYLOGENY OF THE JAW MUSCLES 83 



The loss of the hyomandibular and the degeneration of the opercular 

 bones has caused the loss of the usual muscles of this region of teleosts. 



Amphibia 



rana 



Plate V, Figs. 1, 2 



The fundamental resemblance in the skull of Anura to some of the 

 fossil Amphibia, for example, such as Cacops and Eryops, is quite strik- 

 ing. These are indeed so much like the Anura that in studying them for 

 restoration of the muscles it was found that the musculature demanded 

 was invariably of the anuran type. Cacops in side view shows a type of 

 covered skull that has opened cut to a certain extent in the temporal 

 region, while remaining solid in the dorsal region. Some of the modern 

 Anura, i. e., Pipa americana, show something of the covered condition 

 found in the stegocephalians, giving some likenesses to these fossil forms, 

 although this resemblance may well be secondary. Eryops, also, is very 

 much like the Anura in the palatal region, although the dorsal part of 

 the skull appears to be quite different, because of the complete covering 

 of dermal bones which it has retained from its early fish ancestors. 



The frog skull represents a fenestrated condition of the roofed skull, 

 with. the palatal, postorbital and temporal region cleared of their bony 

 covering. There is a light musculature in the jaw region to meet the 

 conditions of the weak mandibles. It has been suggested (Gregory, 

 1917) that this similarity of the Anura and the stegocephalians is more 

 genetic than convergent and that our Anura may indeed represent certain 

 remnants of stegocephalians in which the roof of the skull became fenes- 

 trated. 



The jaw muscles in the frog, on account of the great open palatal 

 region, are all on the posterior part of the skull, as there is no place for 

 the muscles at the front part of the skull. The jaw muscles of the Anura 

 are given the mammalian names by Ecker and Wiedersheim (1896-1904, 

 pp. 133-136). 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (IXNERVATED BY V 3 ) 



Capiti-mandibularis superficialis. 



(a) Pars major (Masseter major of E. and W.). 



(b) Pars minor (Masseter minor of E. and W.). 

 Capiti-mandibularis profundus (Pterygoideus of E. and W.). 

 Pterygoideus anterior (Temporalis of E. and W.). 



