556 ALLIS. [Vou. XII. 
muscle. It is, in all three, continuous behind with the posterior 
portion of the superficial muscle, and the facial nerve in all pen- 
etrates the muscle at this point ; the point of penetration in 
Heptanchus being at about one third the length of the upper 
jaw, and in Scymnus and Acanthias at its hind end, as in Galeus 
and Carcharinus. In all three fishes the nerve separates at 
once into two parts: a r. hyoideus which runs forward along the 
outer surface of the hyohyoideus, supplying it, mainly (No. 124, 
p. 418), but sending branches also to the superficial muscle, and 
a branch which runs forward along the inner surface of the lower 
jaw and is said, in Scymnus and Acanthias, to supply in part the 
most anterior portion of the constrictor. 
That part of the superficial constrictor that lies in front of 
the point where the muscle is penetrated by the facial nerve 
thus forms an anterior portion, innervated in its anterior part, 
probably, by the trigeminus mainly, but by the facialis in part, 
and in its hindermost portion by the facialis mainly or alone. 
From the anterior part of the muscle the intermandibularis and 
the inferior division of the geniohyoideus probably arise; from 
the next following and hindermost portions the superior division 
of the geniohyoideus. In the several selachians here consid- 
ered and the other fishes described by Vetter a regular transla- 
tion of this last portion of the muscle from before backward is 
shown. In Heptanchus its hindermost fibres extend only to a 
point about two thirds the length of the mandible; in Scymnus 
and Acanthias they have reached the hind end of the lower 
jaw; and in Acipenser they have passed beyond the mandible, 
the muscle arising in part (Cs: and Csz) from a fascia below 
and in front of the eye, an origin most naturally arising from 
the insertion of its superficial bundle in Scymnus, and in part 
(Cs3) from the upper end of the ceratohyal above and behind 
the surface of origin of the hyoideus inferior. In Amia this 
last attachment only is found. In Acipenser the r. maxillaris 
inferior trigemini, after entering the lower jaw, sends two 
branches perpendicularly across the outer surface of Meckel’s 
cartilage (No. 120, p. 233). The anterior of these two branches, 
in the specimen I examined, went to the muscle Cs6 of Vetter, 
and the posterior one backward along the outer surface of Csr 
