No. 3:] A7TOSCLES AND NERVES IN AMIA CALVA. 563 
the inner edge of the mandible to the preopercular articulation, 
and then upward and backward between the two upper poste- 
rior branchiostegal rays, onto the external surface of the gill- 
cover behind and beyond them, where it disappears. The fold 
may be called the hyoideo-mandibular fold. 
In its action as a retractor of the mandible the geniohyoi- 
deus is, as Vetter and others have stated, accessory to the 
sternohyoideus, which is the main retractor of the hyoid and 
through it of the mandible. The sternohyoideus is inserted 
on the under surface of the extreme anterior end of the hyoid 
arch. In its action it pulls this end downward and backward 
in a curved line, so placed, relatively to the curve described 
by the tip of the mandible in opening, that the hyoid not only 
pushes the parts below it downward between the rami of the 
mandible, but, having slipped backward over those parts to the 
full extent allowed by its integumental connection with them 
and with the mandible, has its motion transferred to the man- 
dible, and thus acts as its retractor. The motion so given 
by the sternohyoideus is accelerated and increased by the 
action of the geniohyoideus, both divisions of which muscle, 
by their contraction, tend to hold the mandible down over the 
anterior end of the hyoid so that it shall follow more promptly 
the motion given to the latter. The geniohyoideus also draws 
the hyoid forward, this being its simplest and most direct 
action, but it can do this, and so act as an adductor of the 
hyoid, only when the mandible is fixed by the action of its 
adductor. 
ce. Hyohyoideus. 
The hyohyoideus (Zz and hs, Figs. 43-46, Pls. XXXI 
and XXXII) arises from the median edge of the flat, spreading 
blade of the ceratohyal, from a surface of depression begin- 
ning at the base of the first or second branchiostegal rays and 
extending forward along the ventral side or edge of the shank of 
the ceratohyal, and from the inner surfaces of the branchiostegal 
rays, mainly from the inner surfaces or front edges of the first 
and secondrays. These first two rays do not generally come into 
contact, at their bases, with the ceratohyal, the first ray being 
