No. 3.] MUSCLES AND NERVES IN AMIA CALVA. S72 
inner, posterior surface the front wall of the spiracular canal. 
The two muscles represent, according to Vetter, the dorsal half 
of the superficial constrictor of the mandibular arch. 
In Carcharinus and in Acanthias much the same relations 
exist, but in Scymnus the hind muscle, Csd;, has become 
entirely independent. It arises from the hind, upper corner of 
the skull, and running forward and downward, unquestionably 
along the outer surface of the levator although it is not so 
stated, it encircles the front side of the outer ends of the two 
spiracular cartilages and is inserted on the inner side of the 
quadrate part of the upper jaw. It is called by Vetter the 
muscle of the spiracular cartilage. 
In Galeus a still further differentiation of this muscle has 
taken place, for it is here inserted near the hind edge of the 
lower eyelid, some few fibres only, from its inner surface, leav- 
ing the main muscle to be inserted on the inner surface of the 
upper jaw. A second spiracular muscle is also found. It arises 
immediately behind the spiracular canal from the outer surface 
of, or from tissues superficial to, Csd2. Its lower fibres run 
upward and then forward, partly encircling the spiracle, its 
upper fibres directly forward. Beyond the spiracle the muscle 
divides, allows the larger spiracle muscle to pass through it, 
and then, reuniting, is inserted, according to my notes, near 
the edge of the lower eyelid, at the corner of the eye, above 
and behind the insertion of the larger muscle. In Mustelus, 
also, there are two spiracle muscles. The larger of the two 
is described by Tiesing as the levator palpebrae nictitantes, and 
is unquestionably derived directly from the muscle called by 
Vetter Csd;. The other, smaller, muscle lies internal to the 
larger one, and is inserted on the hind end of the upper eyelid. 
Its insertion is thus higher than that I have noted for the 
corresponding muscle in Galeus. It is called by Tiesing the 
retractor palpebrae superioris, and is undoubtedly, as he con- 
cludes, also derived from Csd1, or perhaps from that muscle 
and Addy also. 
In Heptanchus the anterior part of the adductor mandibulae 
extends almost to the ‘“ palatobasal”’ articular process of the 
upper jaw. In Carcharinus a part of this anterior portion 
