320 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
IV.—MUSCLES OF THE HYOID ARCH. 
1. Gentonyorpeus, (No. 27, Cuv.) (Fig. 3, GH). 
This muscle which runs along the inner side of the ramus of 
the mandible, may be exposed by removing the integument from 
the lower surface of the skull and turning back the mtermandibu- 
laris which covers its insertion. It arises from the posterior 
portion of the lower (ventral) and outer surfaces of the ceratohyal, 
and also from the epihyal at the bases of the upper branchiostegal 
rays. It passes forwards as a thick muscle, inclining slightly in- 
wards towards its fellow of the opposite side, the inner fibres being 
inserted into a median aponeurosis between the two, no interdigita- 
tion occurring. The greater bulk of the muscle inclines outwards, 
and is inserted into the posterior surface of the anterior part of the 
ramus of the mandible, being partly covered by the intermandibu- 
laris. Crossing the anterior portion of the muscle obliquely are two 
tendinous bands, (Fig. 3, ti, ti'), to which are attached the cartilagin- 
ous supports of the tentacles of the under surface. 
TInnervation.—R. hyoideo-mandibularis facialis. 
Action.—According as the hyoid or mandibular arches are fixed 
this muscle acts in different ways. If the hyoid is fixed by the 
hyoclavicularis it acts ou the mandible, depressing it. This is its 
usual action. If, however, the mandible is fixed by the powerful 
add. mand., it raises the hyoid arch and through it the operculum, 
thus aiding the Jev. and dil. operc. Through the tendons which 
pass across it, it is the means by which the tentacles resting on these 
tendons move, but the range of motion thus imparted is very small. 
The simplicity of this muscle contrasts somewhat with what occurs 
in Hsox, and agrees more closely with the arrangement in Barbus. 
In Cyprinus the origin is similar, and in Barbus the muscles of either 
side do not interdigitate as they appear to do in other fishes. In 
Esox and Cyprinus a median enlargement of the muscle occurs. The 
tendinous bands are of course peculiar to the Siluroids. 
2. Hyonyorpeus, (Nos. 28 and 29, Cuv.; Lev. and Dep. branchios- 
tegarum, Ow.) 
This is exposed by the dissection required for the preceding with 
the removal of the integument from the branchiostegal rays. It 
may be considered as being composed of two portions, of which the 
