No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 589 
maera, the remainder of the muscle in Amia, that is, the superior 
division and the median part of the inferior division, must be 
represented by that part of the ventral constrictor which in sela- 
chians lies behind the point where the facial nerve penetrates 
the muscle, with the addition perhaps of some part of the super- 
ficial layer immediately in front of it. This muscle is repre- 
sented in Acipenser by Cs4, with or without the addition of 
Cs3, and in Chimaera by Cs: or Cs1 and Cs2; Css, although lying 
internal to the branchiomandibularis, probably representing 
some part of the geniohyoideus. In both these fishes the 
muscle is still inserted in a median aponeurosis and lies, as it 
does in Amia, external to the coraco-mandibularis, the homo- 
logue of the branchiomandibularis. In Amia the insertion has 
shifted to the under surface of the hypohyal, the muscle of 
either side crossing the middle line of the head and overlapping 
its fellow of the opposite side. 
In the teleosts described by Vetter an apparent reversion or 
retrogression in the hyohyoideus has taken place. The bran- 
chiomandibularis, which in Amia lies between the two parts of 
the muscle at their insertion, has in the teleosts disappeared, 
and the two parts of the hyohyoideus, no longer kept apart by 
it, tend to unite; either that or the anterior and lateral part of 
the muscle gradually disappears. In Esox the tendinous front 
edge of the muscle, found in Amia, has disappeared, and the 
muscle shows at its insertion a distinct separation into two 
parts. In Perca the anterior of these two parts has been 
reduced to a few fibres only, and in Cyprinus and Barbus not 
only have these disappeared, but the attachment to the 
hypohyal, still retained in Perca, has been lost, and the muscle 
is inserted entirely in a median aponeurosis with its fellow of 
the opposite side of the head, or has become continuous with 
that muscle. 
In selachians the hyohyoideus and the hyoid branch of the 
facial that supplies it both lie external to the cartilaginous gill- 
rakers, while in Amia, even in young larvae, and in teleosts, 
both muscle and nerve lie internal to the branchiostegal rays. 
Vetter suggests, in explanation (No. 125, p. §37), that the mus- 
cle has passed from the outer surface of the gill-rakers to the 
