No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 581 
The superficial muscle Az, of Amia, has in teleosts under- 
gone much modification, due mainly to the marked development 
of the superficial portion of the muscle, 42’, and its differentia-. 
tion in some fishes as a wholly separate muscle, the deeper 
parts of the muscle 42" and Az!” undergoing at the same time 
an actual or relative reduction. In Amia the outer fibres of 
A,’ extend at their insertion almost onto a strong ligament, 
which extends from the coronoid process of the mandible to 
the inner surface of the maxilla. A slight change in the inser- 
tion of these fibres would give rise to the condition found in 
Perca, where the muscle A1, of Vetter, is inserted in part 
by tendon on the inner surface of the maxilla, and in part has 
retained its connection with the deeper fibres of the adductor, 
and is inserted with them on the anterior, lower corner of the 
articular. In Barbus this last connection has disappeared, and 
the muscle is inserted entirely on the maxilla along its lower 
edge. In Cyprinus the muscle has separated into two portions, 
wholly distinct at their insertions on the maxilla, but still 
united at their origins, while in Moxostoma, which I examined, 
the separation at the origin has become complete, and each of 
the two muscles is again partly separated into two portions. In 
Moxostoma the deeper muscle Az of Vetter, which is equiva- 
lent to 42” and A2'” in Amia, is greatly reduced, and is simply 
a thick muscle band lying along the upper edge of A3. That 
this band is the muscle A2 of Vetter, or a part of it, and not 
a part of A, which it has every appearance of being, is evident 
from the position of the r. ad musc. adductor mandibulae, 
which runs under the band and issues between it and 43, the 
remaining lower portion of the muscle ; for it is not probable 
that this nerve, which lies so constantly between Az and A; in 
other fishes, should have so radically shifted its position in this 
one as to penetrate A; from its under surface. 
In Esox and Amiurus more primitive arrangements are pre- 
sented than that found in Amia, for in neither of them is there 
any separation of a superficial layer, although there are in both 
indications either of the absorption, or of the preéxistence, of 
such a layer. In determining the existence of this layer, the 
course and position of the inferior maxillary nerve is of first 
