No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 587 
and Cs2. Whether or not it extended to Cs3 and Cs4 I did 
not attempt to determine, nor did I attempt to trace the course 
of the facial nerve. It was sufficient for my purpose at the time 
to know that Csr and Csz represent, in all probability, the supe- 
rior division of the geniohyoideus of Amia, and that Cs3 either 
represents also a part of that muscle or that, continuing its 
backward translation, it has finally lost its attachment to the 
upper end of the ceratohyal and become associated with Cs4, 
probably as part of the superior division of the hyohyoideus, 
Css representing, in part at least, the inferior division of that 
muscle. 
As the lateral ends of that part of the constrictor that gives 
rise to the superior division of the geniohyoideus thus travelled 
backward relatively to the mandible, the median ventral portion 
of the entire muscle seems to have travelled in the same direc- 
tion, and relatively at a still more rapid rate, thus giving rise 
to the backward and inward direction of the fibres seen in the 
entire constrictor in Scymnus and Acanthias, in the mylohyoi- 
deus in Acipenser, and in the inferior geniohyoideus in Amia. 
The backward translation of this median part of the muscle 
then apparently ceased, and a translation relatively forward of 
the median end of the superior geniohyoideus alone began, 
giving rise to the condition in Acipenser, where the median 
portion of the superior muscle, Csi and Csz2, overlaps externally 
the inferior muscle, Cs6, and to the condition in Amia where 
the superior division of the muscle extends in part internal to 
the inferior division and is inserted in the median aponeurosis, 
on the hind edge of the intermandibularis, on the floor of the 
mouth, or on the mandible near the symphysis. 
In teleosts the inferior geniohyoideus is gradually absorbed 
by the superior muscle and finally disappears entirely. The 
first stage in this process is shown in Esox, where the fibres of 
the inferior division, although retaining their special origin on 
the mandible, have become at their insertion directly contin- 
uous with fibres corresponding to the median, superficial portion 
of the superior muscle in Amia. In Perca and Cyprinus the 
origin of the fibres of the inferior muscle on the mandible has 
disappeared, but its insertion is still in a measure retained, for 
