584 ALLIS. [Vou. XII. 
there be such a branch, it must correspond to the nerve that I 
shall describe in Amia as r. ghi, and the important branch in 
Galeus and Carcharinus to the nerve r. ghs ; if there be no such 
branch, and if there be no other branch sent to the constrictor 
proximal to the one that I have described in Galeus and Car- 
charinus, then this one branch alone must give rise to the two 
branches in Amia, one of its branches splitting off from the 
main nerve as the regions innervated by the branches became 
distinct and separate. 
Whatever the homologies of these several branches of the 
trigeminus may be, their distribution in selachians shows 
that a large and important part of the ventral constrictor in 
those fishes may belong to the mandibular arch. Vetter’s 
muscle Csvz, therefore corresponds, in its innervation partly by 
the trigeminus and partly by the facialis, to the three muscles 
described under this subheading in Amia. As the muscle in 
selachians, in its superficial portion, forms an unbroken layer 
extending from near the symphysis to and beyond the hind end 
of the lower jaw, the determination, even in a general way, of 
which part of it gives rise to each of the three muscles in Amia 
is largely a matter of supposition. 
In Galeus and Carcharinus there is no superficial bundle 
of Csvz inserted on the outer surface of the adductor mandi- 
bulae, as described by Vetter in Scymnus and Acanthias. The 
hindermost fibres, however, of that part of the muscle that is 
inserted along the lower edge of the mandible, and which may 
be called the anterior portion of the muscle, form a somewhat 
separate bundle, continuous with the rest of the muscle in its 
median ventral portion, but distinctly separate from it at its 
insertion. This bundle is also distinctly separate at its inser- 
tion from that part of the constrictor that lies immediately 
posterior to it. When its fibres are separated with the scalpel 
from the rest of the muscle, an inner, deeper layer is disclosed 
lying immediately internal to it. This deeper layer has its 
origin entirely from the hyoid arch, and is continuous behind 
with the posterior portion of the superficial constrictor. It ex- 
tends forward about one half the length of the mandible, runs 
almost directly medianward to the lateral edge of the m. coraco- 
