862 
U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY-GENERAL REPORT. 
There are sharks in which the eyelid is provided with a nictitating membrane, and others in 
which this peculiarity of structure does not exist. The same may he stated with regard to the 
spiracles. As far as observations go, the spiracles and nictitating membrane are never wanting 
in the same group; whilst both the spiracles and the nictitating membrane may exist simulta¬ 
neously. Sometimes we observe spiracles and no nictitating membrane; at others, the spiracles 
are wanting, while the nictitating membrane is extant. 
Family SCYLLIODONTIDAE^ Girard. 
The head is depressed, the snout obtuse, the nasal flap or valve broad, and rather elongated; 
the grooves at the angle of the mouth being large, and the lips cartilaginous; the eyelids 
constituting an elongated cleft. Tlie spiracles are of medium size. The teeth, which resemble 
those of the genus Scyllium, exhibit nevertheless a more elongated central prong, with several 
lateral and smaller ones at the base. The general aspect of the fins is similar also to that 
of Scyllium. The inferior lobe of the caudal fin is likewise atrophied, and the caudal furrow 
wanting. The scales are tri-digitated and triply keeled. 
Syn _ Scylliodonles, Mull. & Henle', Syst. Beschr. Plagiost. 1841, 63.—Dum. Ichthyol. analyt. 1856, 130. 
Scylliodontini, Bd. Iconogr. Encycl. IF, 1850, 241. 
The Scylliodonts belong to that division of the sharks in which there are two dorsal fins and 
one anal, the first dorsal being placed between the pectorals and the ventrals. They are pro¬ 
vided with spiracles and a nictitating membrane, the last or last two branchial fissures being 
situated above the base of the pectoral fins. 
TRIAKlI Mull. & Henle. 
Gen. Char. —This genus being the only one of its group, its diagnostic characters may be deduced from those of the family 
just alluded to. The teeth are rather small and numerous. There is one central fang or prong directed obliquely outwards 
with one or several small ones on either side of the base. 
Syn. — Triakis, Moll. & Henle, Syst. Beschr. Plagiost. 1841, 63.— Dum. Ichthyol. analyt. 1856, 130. 
So far but one species was on record, an inhabitant of the Japanese seas. The one whose 
description follows is closely allied to it, either of them apparently not reaching any very 
large size. 
TRIAKIS SEMIFASCIATUS, Grd. 
Spec. Char —Head very much depressed, constituting the sixth of the total length. Back rather arched ; caudal region 
very much tapering. Shagreen with very acute acerated points. Color above olivaceous grey with tranverse black bands and 
rounded spots ; beneath yellowish, unicolor. 
Syn. — Triakis semifasciatum, Grd. in Proc. Acad. Nat. Sc. Philad. VII, 1854, 196. 
The general aspect of this species is elongated. In the specimen before us, and which mea¬ 
sures twelve and two-thirds of an inch, the head, from the extremity of the snout to the first 
branchial aperture, constitutes the sixth of the total length. The back is rather more arched or 
convex than in T. scyllium , the caudal region being very slender and tapering. 
The head is very much depressed, sub-convex and sloping sideways and forwards. The snout 
is rounded off, and seen either from above or from below its anterior outline is semi-elliptical; 
the nostrils being somewhat nearer the mouth than the extremity of the snout. The flap or 
valve at their anterior edge is placed more towards the inner than the outer angle ; it is taper¬ 
ing, rounded off, and directed obliquely outwards and backwards. The outline of the mouth is 
semi-elliptical; its width, measured from angle to angle, is equal to the rostral distance between 
