ADAMS, PHYLOGENY OF THE JAW MUSCLES 115 



DIDELPHYS 

 Plate X, Figs. 1, 2 



The skull of Didelphys is remarkably similar to that of lower Eocene 

 Carnivora in so far as it has a very narrow brain case, high sagittal crest, 

 long heavy muzzle, stout zygomata and stout curved mandibles. The 

 pterygoid bones are reduced, as is frequently the case in carnivorous 

 mammals. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V s ) 



Masseter Pterygoideus interims 



Temporalis Digastricus (pars anterior) 



Pterygoideus externus 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Digastricus (pars posterior) 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 {INNERVATED BY F a ) 



Masseter. — This is a very large and strong muscle, arising on the 

 under side of the zygomatic arch and from the fascia extending up over 

 the temporal muscle. The insertion is on the posterior, ventral part of 

 the angle of the mandible. 



Temporalis. — This is a large muscle filling the temporal fossa and ex- 

 tending to the high sagittal crest. The insertion is on the coronoid 

 process of the mandible. 



Pterygoideus externus. — Arises behind the pterygoideus internus on 

 the alisphenoid and is inserted on the inner side of the mandible. 



Pterygoideus internus. — Arises on the pterygoid wing of the alisphe- 

 noid and is inserted on the inner side of the angle of the mandible. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Digastricus posterior. — This is quite a large muscle, arising on the 

 mastoid and inserted on the inner side of the mandible, well forward. 



