Fig. 321.—Fin-spine of Hybodus reticulatus Agassiz. (After Zittel.)
Palæospinax, with short stout spines and very large pectoral fins, formerly regarded as a dogfish, is placed near Heterodontus by Woodward. Acrodus, from the Triassic, shows considerable resemblance to Heterodontus. Its teeth are rounded and without cusps.
Most of these species belong to the Carboniferous, Triassic, and Jurassic, although some fragments ascribed to Cestraciont sharks occur in the Upper Silurian. Asteracanthus, known only from fin-spines in the Jura, probably belongs here.
It is a singular fact first noted by Dr. Hay, that with all the great variety of sharks, ten families in the Carboniferous age, representatives of but one family, Heterodontidæ, are found in the Triassic. This family may be the parent of all subsequent sharks and rays, six families of these appearing in the Jurassic and many more in the Cretaceous.
Edestus and its Allies.—Certain monstrous structures, hitherto thought to be fin-spines, are now shown by Dr. Eastman and others to be coalescent teeth of Cestraciont sharks.
Fig. 322.—Fin-spine of Hybodus canaliculatus Agassiz.
Fig. 323.—Teeth of Cestraciont Sharks. (After Woodward.) a, Hybodus lævis Woodward (after Woodward); b, Heterodontus rugosus Agassiz; c, Hybodus delabechei Charlesworth.