38 BULLETIN OF THE BUREAU OF FISHERIES. 



LEVATOR OPERCULI. 



The levator operculi is a short, triangular muscle which arises from the posterior 

 spinous border of the pterotic. The fibers converge diagonally downward and back- 

 ward to an insertion in the upper margin of the opercle. Its contraction leads to an 

 elevation of the opercle aiding in the act of respiration. 



DEEP DORSAL HEAD MUSCLES. 



When the eye is removed along with the upper portion of the metapterygoid and 

 hyomandibular bones a broad curved sheet of muscle consisting of short thick bundles 

 is exposed. The homologous mass in A mia has been divided by Allis into three portions — 

 the levator maxillae superioris, the adductor hyomandibularis, and the adductor operculi. 

 In Oncorhynchus this region can scarcely be di^dded except for the small group of fibers 

 at the posterior limit of the region. The two parts are better identified under McMurrich's 

 names, the adductor arcus palatini, and the adductor operculi. 



ADDUCTOR OPERCULI. 



The adductor operculi arises on the ventral surface of the pterotic directly under 

 the origin of the levator operculi. Its origin is overlapped by the posterior fibers of the 

 adductor arcus palatini. The fibers form a short but thick bundle, its length being from 

 8 to lo mm. It is inserted into the inner surface of the opercle a little above the inser- 

 tion of the dilator operculi which it opposes in action. 



ADDUCTOR ARCUS PALATINI. 



This muscle has an extensive origin along a line from the origin of the adductor 

 operculi to a point in the ventral portion of the eye socket. The fibers are short, thick, 

 and massive for the position in which they lie, but are not readily separated into distinct 

 bundles. 



The posterior half of this muscle arises just ventral to the articulation of the hyo- 

 mandibular and from the ventral surface of the pterotic and sphenotic bones. The 

 fibers are only a few millimeters in length, run directly outward, and are attached into 

 the inner surfaces of the upper half of the hyomandibular and the posterior portion of 

 the metapterygoid. The anterior portion of the mass, which is relatively the larger, 

 arises from the outer surface of the orbitosphenoid. Its fibers extend outward and 

 downward to a broad attachment on the inner surface of the metapterygoid and the 

 dorsal surface of the mesopterygoid. 



The contractions of the entire mass tend to elevate the angle of the jaw and to 

 compress the palatine arch. 



Allis ° di\'ides this muscle into the levator maxillae superioris and the adductor 

 hyomandibularis. No natural division along these lines can be obsen^ed in Oncorhynchus 

 tschawyischa. 



DORSAL BRANCHIAL ARCH MUSCLES 



When the palatine arch is removed and the adductor arcus palatini reflected, one can, 

 by trimming away the gill filaments, readily expose the group of muscles of the dorsal 

 half of the branchial arches. 



a Allis. Edward Phelps, loc. cit. 



