THE MYOLOGY OF AMIURUS CATUS. 321 



posterior belongs essentially to the branchiostegal rays. This por- 

 tion (Fig. 3, Hh 1 ) arises from the inner surfaces of the operculum 

 and interoperculum, extending from them to the dorsal border of the 

 first branchiostegal ray Thence it passes below that ray to the dor- 

 sal border of the second, and so on to the most internal ray, becom- 

 ing narrower as it nears the median line, and having its central fibres 

 better developed than the lateral ones. From the last ray the 

 muscle extends upwards and forwards, and is inserted into the apon- 

 eurosis which separates it from its fellow. 



The anterior portion (Fig. 3, Hh 2 ), arises from the upper border 

 and surface of the ceratohyal and hypohyal, and passing inwards is 

 inserted into the aponeurosis between it and its fellow. 



Innervation. — R. hyoideo-mandibularis facialis. 



Action. — Both portions act as constrictors. The posterior portion 

 will close the aperture of the gill cavity by shutting down upon it 

 the branchiostegal membrane. The complete closure of the "gill- 

 slit" is necessary in order that the hyoid apparatus and its muscles 

 may properly perform their pumping action. The anterior portion 

 approximates the hyoid arches, and thus aids the posterior portion, 

 drawing the whole hyoid apparatus towards the side of the skull. 



The hyohyoideus varies somewhat in different forms. In Esox it 

 passes as a continuous sheet over the branchiostegal rays, not passing 

 from one to the other as in Amiurus and the Cyprinoids. In Perca 

 and Esox the muscle passes directly across to the hyoid arch of the 

 opposite side, and in the latter there is a separation into two bundles 

 of which the outermost passes forward and is inserted into the cera- 

 tohyal and hypohyal, and therefore corresponds to the anterior 

 muscle of Amiurus. In Perca neither Stannius nor Cuvier nor Owen 

 describes an anterior portion, but Owen states 1 that " In some fishes 

 a transverse muscle, repeating the characters of 21-, Fig. 135, (i.e., 

 the intermandibularis), passes from one ceratohyal to the other." Vet- 

 ter terms that portion of the muscle which runs between the branchi- 

 ostegal rays the ' hyohyoideus superior,' grouping those portions 

 coming from the most internal ray and from the ceratohyal together 

 as the ' hyohyoideus inferior' an arrangement which in Esox is 

 quite proper, but will not hold with Amiurus. 



l Owt>,n. —hoc. eit. 



