562 MELTS. [Vor XI. 
already described, it meets the posterior bundle of fibres of 
the deeper muscle. 
The two divisions of the geniohyoideus are intimately con- 
nected at and near their insertion on the median aponeurosis, 
but the fibres of the two portions lie at a considerable angle to 
each other, and the continuity described by McMurrich was not 
found in any specimen, old or young. 
In larvae 10 mm. in length some of the fibres of the super- 
ficial portion of the muscle are continuous with those of the 
intermandibularis, the fibres of the left side being continuous 
with the ventral layer of the intermandibularis, and those on 
the right side with the dorsal one. In these young larvae the 
geniohyoideus is also apparently continuous with the hyohyoi- 
deus. 
The geniohyoideus acts either as an adductor of the hyoid 
arch or as a retractor of the mandible according as the one or 
the other of those structures is stationary, or is so considered. 
The hyoid arch is loosely connected, in front, with the inner 
surface of the mandible by a fold of the tough integument of 
the floor of the mouth, which, when the parts are at rest, is 
folded under the tip of the hyoid, thus forming the covering of 
its under surface and the floor of the anterior part of the mouth. 
Back of this, along each side, the hyoid is connected with the 
lower, inner edge of the mandible by a strong fold formed by 
the closely united external dermis and internal lining membrane 
of the mouth, the two membranes being so closely united that 
they are with difficulty separated. A strong membranous fold 
is thus formed, which, when the mouth parts are at rest, lies 
directed upward, from the lower edge of the mandible, parallel 
to and against its inner surface. By this arrangement the 
hyoid arch, lying between the rami of the mandible, is con- 
nected with them, on either side, by a long and flexible fold, so 
that its anterior end, which is smaller than the space between 
the rami, can be freely elevated and depressed between those 
rami to the full extent permitted by the fold. A deep groove 
(No. 3, Fig. 16, Pl. XXXV and Fig. 22, Pl. XXXVI) on the 
under surface of the head marks the position of this fold, 
the groove extending from near the symphysis backward along 
