ADAMS, PHYLOGENY OF THE JAW MUSCLES 89 



of the skull covered and that there never has been a representative of this 

 group with any fenestration of the lizard type. They have adopted a 

 different scheme for the opening of the temporal muscle region by the 

 posterior excision of the temporal roof, leaving the muscles of the head 

 more or less bare. Except posteriorly, the roof is complete in Chelone 

 and there is a progressive reduction of the roof in the modern forms 

 through Chelydra and Trionyx, until finally the opening is complete in 

 Cistudo, where even the jugal has lost its. hold on the quadrate. These 

 forms thus give a morphological series. The reduction of the temporal 

 roof is analogous to the fenestration of the skull in other orders of rep- 

 tiles, where the temporal roof is perforated dorsally and laterally. 



The great occipital crest indicates a corresponding temporal muscle in 

 Chelonia, where the muscle extends behind the condyle and back over 

 the vertebra? of the neck. The quick snap of the jaws of Chelydra and 

 Trionyx is due to the extension of the occipital crest which gives the 

 temporal muscle a long origin and an oblique angle of insertion. The 

 great separation of the squamosal from the parietal causes the depressor 

 mandibulae to slip down from its usual origin on the parietal to the out- 

 wardly projecting squamosal. 



The turtle with its monimostylic skull has very simple jaw muscles, 

 only three muscles being differentiated. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INXERVATED BY V s ) 



Capiti-mandibularis (Cm.). 



= M. Occipito-squamoso-maxillaris, Hoffmann (1890). 



= Schlafenmuskeln, Wiedemann. 



= Temporalis, Bojanus, Stannius, Cuvier, Owen. 

 Pterygoideus anterior (Pt. a.). 



= Pterygo-maxillaris, Hoffmann. 



= M. Pterygoideus, Bojanus, Owen. 



= Fliigelmuskel, Wiedemann. 



= Pterygoideus internus, Stannius. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (IXXERVATED BY VII) 



Depressor mandibulae (D. m.). 



= Digastricus maxilla?, Bojanus. 



= Squamoso-maxillaris, Hoffmann. 



= Masseter (apertor oris), Wiedemann. 



= Apertor oris, Owen. 



= Senker des unterkiefers, Stannius. 



