ADA3I8, PHYLOGENY OF THE JAW MUSCLES 71 



(c) Pars temporalis medialis (Ad. m.^) is the inner slip of the medial 

 layer pars temporalis lateralis. 



(d) Pars intramandibularis (Ad. m.*, Ao)Vetter) is the part of the 

 adductor mass that extends forward into the hollow ramus of the jaw as 

 in the teleosts. It arises from the fascia on the other two parts of the 

 adductor and fills the hollow ramus, being attached to the splenial and 

 the dentary. 



Protractor hyomandihularis. — This muscle is not present in Amia as 

 a separate muscle, but its function is taken by the posterior part of the 

 levator arcus palatini, this has a small insertion on the underside of the 

 hyomandibular, thus acting as a protractor of that element. 



Dilator operculi (D. o.). — This arises on the pterotic, extends through 

 the- usual fossa in the hyomandibular and is inserted on the anterior, 

 inner face of the operculum by a tendon. The path of this muscle over 

 the hyomandibular, just above the preoperculum is well marked, as it is 

 in all fishes. This muscle raises and pulls the opercular region outwards. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



These muscles of the facialis group represent the C^sd of Vetter, that 

 is the dorsal part of the "second constrictor^' pertaining to the hyoid arch 

 (Cosd). 



Adductor hyomandihularis (Ad. h.). — This arises from the otic region 

 and is inserted on the inner face of the hyomandibular, along the posterior 

 edge to the articulation for the operculum. It pulls the hyomandibular 

 dorsally. 



Adductor operculi (Ad. o.). 



Levator operculi (L. o.).— These two muscles of the operculum both 

 arise on the pterotic and parietal, the adductor operculi being the anterior 

 slip. They extend down to the operculum and spread out on its inner 

 face. They raise the operculum. 



LEPIDOSTEUS 

 Plate II, Fig. 3 



Lepidosteus has a well developed skull with a great elongation of the 

 mandible and maxillse. The great forward prolongation of the quadrate 

 and mandible gives what is demanded for a catcher of fish, namely, a 

 quick snap. The enormous adductor with its temporal and masseter 

 slips must serve to close the jaw with a very quick snap, so that the 

 numerous sharp pointed teeth get a hold on the slippery prey. These 



