68 ANNALS NEW YORK ACADEMY OF SCIENCES 



extends to the hyomandibular and is inserted along its anterior margin, 

 with a long insertion extending almost to the distal end of the bone. 

 It protracts the hyomandibular, pulling the distal end outwards and 

 forward. The homology of this muscle is interesting. Danforth (1913) 

 says : "The partial division of this muscle is of some interest, since the 

 homologies of the levator arcuus palatini and the dilator operculi in the 

 teleosts are rather uncertain." 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Retractor hyomandibularis et operculi (levator and add. operculi?) 

 (E. hy. et o.). — This is a large sheet of muscle that arises along the 

 groove in the cranium, under the long pterotic. It may represent the 

 levator operculi and the adductor operculi of Amia. The fibers spread 

 out and are divided into small bundles and are attached to the posterior 

 edge of the hyomandibular. Posteriorly they attach themselves to the 

 whole dorsal border of the degenerate operculum. A few superficial 

 fibers extend over this sheet from the anterior part of the origin to the 

 operculum. These muscles raise the hyomandibular and operculum. 



ACIPENSER 

 Plate I, Fig. 3 



The dermal plates of the skull are superficial ossifications. The carti- 

 laginous skull of this form is more degenerate than in Polyodon, as the 

 mouth is reduced to a small sucking disc, while Polyodon has duck-like 

 jaws. The adductor is shrunken to a small, short muscle, as there is very 

 little demand for a strong muscular action. The opercular and hyoman- 

 dibular region have large muscles, as there is much movement in these 

 parts during the ingestion of food. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V 3 ) 



(1) Adductor mandibular (Ad. m.). 



(2) Protractor hyomandibulge (P. h.). 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



(1) Eetractor hyomandibularis (E. hy.). 



(2) Levator operculi (L. o.). 



