Fig. 335.—Cub-shark, Carcharias lamia Rafinesque. Florida.
A closely related genus is Prionace, its species Prionace glauca, the great blue shark, being slender and swift, with the dorsal farther back than in Carcharias. Of the remaining genera the most important is Scoliodon, small sharks with oblique teeth which have no serrature. One of these, Scoliodon terræ-novæ, is the common sharp-nosed shark of our Carolina coast. Fossil teeth representing nearly all of these genera are common in Tertiary rocks.
Probably allied to the Carchariidæ is the genus Corax, containing large extinct sharks of the Cretaceous with broadtriangular serrate teeth, very massive in substance, and without denticles. As only the teeth are known, the actual relations of the several species of Corax are not certainly known, and they may belong to the Lamnidæ.
Fig. 336.—Teeth of Corax pristodontus.
Family Sphyrnidæ, or Hammer-head Sharks.—The Sphyrnidæ, or hammer-headed sharks, are exactly like the Carchariidæ except that the sides of the head are produced, so as to give it the shape of a hammer or of a kidney, the eye being on the produced outer edge. The species are few, but mostly widely distributed; rather large, voracious sharks with small sharp teeth.
The true hammer-head, Sphyrna zygæna, Fig. 337, is common from the Mediterranean to Cape Cod, California, Hawaii, and Japan. The singular form of its head is one of the most extraordinary modifications shown among fishes. The bonnet-head (Sphyrna tiburo) has the head kidney-shaped or crescent-shaped. It is a smaller fish, but much the same in distribution and habits. Intermediate forms occur, so that with all the actual differences we must place the Sphyrnidæ all in one genus. Fossil hammer-heads occur in the Miocene, but their teeth are scarcely different from those of Carcharias. Sphyrna prisca, described by Agassiz, is the primeval species.
The Order of Tectospondyli.—The sharks and rays having no anal fin and with the calcareous lamellæ arranged in one or more rings around a central axis constitute a natural group to which, following Woodward, we may apply the name of Tectospondyli. The Cyclospondyli (Squalidæ, etc.) with one ring only of calcareous lamellæ may be included in this order, as also the rays, which have tectospondylous vertebræ and differ from the sharks as a group only in having the gill-openings relegated to the lower side by the expansion of the pectoral fins. The group of rays and Hasse's order of Cyclospondyli we may consider each as a suborder of Tectospondyli. The origin of this group is probably to be found in or near the Cestraciontes, as the strong dorsal spines of the Squalidæ resemble those of the Heterodontidæ.
