GREGORY: FISH SKULLS 



191 



The thrusting forward of tlie upper lip is, according to Edwards, effected partly by the 

 outward and forward swing of the entire upper jaw and suspensorium, which also carries 

 with it the mandible. This movement is caused by the contraction of the strong protractor 

 hyomandibularis muscle. Thus the ascending rami of the opposite dentaries push the 

 lower ends of the maxillae before them, rocking the upper ends of the maxillae backward 

 and apparently releasing the premaxillae. 



Labeo 



Fig. 75. Comparison of (A) Laheo sp. and (B) Hy'drocyon. Top views. 



Food is ingested by suction through the alternate enlargement and constriction of the 

 oral cavity. Expansion is accomplished largely by the cooperation of the protractor 

 hyomandibularis and of the levator arcus palatini muscles with the ventral muscles of the 

 mandible, contraction, by the adductor hyomandibularis, intermandibularis and adductor 

 mandibulae (Edwards, 1926). By stimulating the geniohyoideus and sternohyoideus of 

 the live fish, Edwards (p. 268) obtained a forward and downward thrusting of the pre- 

 maxillae. On the other hand, when the adductor mandibulae was stimulated the jaws 

 were retracted. 



