Closely allied to Scyllium is Pristiurus, from the coasts of Europe, which is provided with a series of small flat spines on each side of the upper edge of the caudal fin.
Fossil forms of Dog-fishes are not scarce in the Lias and Chalk: Scylliodus, Palæoscyllium, Thyellina, Pristiurus.
Ginglymostoma.—The second dorsal fin opposite to, and somewhat in advance of, the anal. Eyes very small; spiracle minute and behind the eye. Nasal and buccal cavities confluent. The nasal valves of both sides form one quadrangular flap in front of the mouth, each being provided with a free cylindrical cirrhus. The fourth and fifth gill-openings are close together. The teeth stand either in many series, each having a strong median cusp and one or two smaller ones on each side (Ginglymostoma), or they stand in a few (three) series only, the foremost only being in function, and each tooth having a convex, finely and equally serrated margin (Nebrius).
Four species from the tropical parts of the Atlantic and Indian Oceans, attaining to a length of some 12 feet. Pelagic.
Stegostoma.—The first dorsal above the ventrals, the second in advance of the anal, which is very close to the caudal. Tail, with the caudal fin, exceedingly long, measuring one-half of the total length. Eyes very small, spiracle as wide as, and situated behind, the orbit. Nasal and buccal cavities confluent. Snout very obtuse; upper lip very thick, like a pad, bent downwards over the mouth, with a free cylindrical cirrhus on each side. Teeth small, trilobed, in many series, occupying in both jaws a transverse flat subquadrangular patch. The fourth and fifth gill-openings are close together.
The single species (St. tigrinum) for which this genus has been formed, is one of the commonest and handsomest sharks of the Indian Ocean. Young individuals keep generally close to the coasts, whilst the adult, which are from 10 to 15 feet long, are not rarely met in the open ocean. The colour is a brownish yellow, ornamented with black or brown transverse bands, or with snuff-coloured rounded spots; hence this shark is frequently mentioned by the names of “Zebra-Shark” or “Tiger-Shark.”
Fig. 116.—Chiloscyllium trispeculare, from North-western Australia.
Chiloscyllium.—The first dorsal fin above or behind the ventrals. Anal fin placed far behind the second dorsal, and very close to the caudal. Spiracle very distinct, below the eye. Nasal and buccal cavities confluent. Nasal valve folded, with a cirrhus. Teeth small, triangular, with or without lateral cusps. The two last gill-openings close together.
“Dog-fishes,” from the Indian Ocean, of small size. Four species are known, of which one, Ch. indicum, is one of the commonest shore-fishes on the coasts of this region, extending from the southern extremity of the African Continent to Japan.