ADAMS, PHYLOGENY OF THE JAW MUSCLES 143 



nected with the protractor operculi, always lying just behind it, extend- 

 ing over the groove for it in the hyomandibular just above the preoper- 

 cular. Often the preopercular also shows where the muscle extends over 

 the upper end of it. Practically every skull studied showed this muscle 

 area, so that it is easily demonstrated on any skull in the teleosts or other 

 fish group. It is inserted on the under side of the opercular in most 

 forms. 



Adductor mandibulce. — This is considered as the mother mass of the 

 chief jaw muscles throughout the vertebrates. In the elasmobranchs it is 

 almost a single mass, partially subdivided in the rest of the fishes and 

 reptiles and completely divided in the Amphibia and Mammalia. It is 

 innervated by the third branch of the trigeminus and represents a part 

 of the first constrictor with its nerve. The condition in the elasmo- 

 branchs is usually as a single mass, although the fibers cross each other 

 and do not always extend in the same direction, but there is no definite 

 separation in those studied and none described in the literature on other 

 forms. There is a tendency throughout the remaining Pisces for this 

 mass to be partially divided. In general this muscle may be divided into 

 two parts — a superficial part that extends across the other fibers and is 

 attached in the quadrate and squamosal region and a deeper set of fibers 

 which extend up to the postfrontal and parietal region. No attempt is 

 made in this paper to homologize these portions, although they are con- 

 stant and seen to be starting a condition that eventually may have re- 

 sulted in the separation of these slips from the mother mass. The sim- 

 plest adductor mandibular is found in Acanthias and the most complex 

 in Amia and Esox, although the division is never complete. Amia, Pali- 

 nurichthys and many other forms show a peculiar specialization, where 

 a part of the adductor (Adm 4 ) extends into the suprameckelian fossa and 

 excavates the dentary, so that the bone is completely filled with this part 

 of the muscle. 



MUSCLES INNERVATED BY THE FACIALIS NERVE 



This series of three muscles is concerned with the movement of the 

 opercular and hyomandibular bones. They represent a part of the second 

 dorsal constrictor and show the characteristic innervation of this mass. 



Adductor hyomandibularis. — This is a deep muscle which arises on the 

 posterior part of the skull in the otic region, anterior to the other two. 

 It is usually small and short and is not easily seen unless the opercular 

 bones are removed. It opposes the protractor hyomandibularis and raises 

 the hyomandibular bone. It is attached to the inner side or to the pos- 

 terior border. It is shown in Acipenser, Polyodon and Polypterus. 



