Fig. 324.—Edestus vorax Leidig, supposed to be a whorl of teeth. (After Newberry.)
These remarkable Ichthyodorulites are characteristic structures of sharks of unknown nature, but probably related to the Heterodontidæ. Of these the principal genera are Edestus, Helicoprion, and Campyloprion. Karpinsky regards these ornate serrated spiral structures as whorls of unshed teeth cemented together and extending outside the mouth, "sharp, piercing teeth which were never shed but became fused in whorls as the animals grew."
Dr. Eastman has, however, shown that these supposed teeth of Edestus are much like those of the Cochliodontidæ, and the animals which bore them should doubtless find their place among the Cestraciont sharks, perhaps within the family of Heterodontidæ.
Fig. 325.—Helicoprion bessonowi Karpinsky. Teeth from the Permian of Krasnoufimsk, Russia. (After Karpinsky.)
Onchus.—The name Onchus was applied by Agassiz to small laterally compressed spines, their sides ornamented with smooth or faintly crenulated longitudinal ridges, and with no denticles behind. Very likely these belonged to extinct Cestraciont sharks. Onchus murchisoni and Onchus tenuistriatus occur in the Upper Silurian rocks of England, in the lowest strata in which sharks have been found.
To a hypothetical group of primitive sharks Dr. Hasse has given the name of Polyospondyli. In these supposed ancestral sharks the vertebræ were without any ossification, a simple notochord, possibly swollen at intervals. The dorsal fin was single and long, a fold of skin with perhaps a single spine as an anterior support. The teeth must have been modified dermal papillæ, each probably with many cusps. Probably seven gill-openings were developed, and the tail was diphycercal, ending in a straight point. The finely striated fin-spines not curved upward at tip, called Onchus from the Upper Silurian of the Ludlow shales of England and elsewhere, are placed by Hasse near his Polyspondylous sharks. Such spines have been retained by the group of Chimæras, supposed to be derived from the ancestors of Onchus, as well as by the Heterodontidæ and Squalidæ.
Family Cochliodontidæ.—Another ancient family known from teeth alone is that of Cochliodontidæ. These teeth resemble those of the Heterodontidæ, but are more highly specialized. The form of the body is unknown, and the animals may have been rays rather than sharks. Eastman leaves them near the Petalodontidæ, which group of supposed rays shows a similar dentition. The teeth are convex in form, strongly arched, hollowed at base, and often marked by ridges or folds, being without sharp cusps. In each jaw is a strong posterior tooth with smaller teeth about. The elaborate specialization of these ancient teeth for crushing or grinding shells is very remarkable. The species are chiefly confined to rocks of the Carboniferous age. Among the principal genera are Helodus, Psephodus, Sandalodus, Venustodus, Xystrodus, Deltodus, Pœcilodus, and Cochliodus.
