Fam. 1. Ptyctodontidae.—This Palaeozoic family is known only by the dental plates, of which there is a single pair in each jaw, meeting at the symphysis. Ptyctodus[[550]] and Rhynchodus occur in the Devonian of either Russia or Germany, and in North America, and Palaeomylus only in the Devonian of North America.
Fam. 2. Squaloraiidae.—General shape of the body similar to the existing Harriotta. There is a long, depressed, preoral rostrum, and in the male the head carries a long slender frontal spine. Conical denticles are sparsely present on the head and body. No dorsal fin-spine. Dental plates similar to those of the living Chimaeroids, but thinner, the tritoral areas being less well defined. The only genus is Squaloraia from the English Lias, of which nearly complete skeletons are known.[[551]]
Fam. 3. Myriacanthidae.[[552]]—Body elongate, but less depressed. A dorsal fin-spine is present, and in the males a frontal spine. The dentition consists of a median incisor-like tooth at the symphysis of the lower jaw, in addition to dental plates similar to those of Squaloraia. There is a symmetrical series of tuberculated dermal plates on the lateral surfaces of the head, which probably represent groups of fused denticles. One species (Myriacanthus granulatus) has its rostrum terminating in a cutaneous flap, as in Callorhynchus. Myriacanthus, from the Lower Lias of Lyme Regis, and Chimaeropsis, from the Lithographic Stone of Bavaria, are the only two genera.
Fam. 4. Chimaeridae.—Body elongate and shark-like in form, but the head is compressed and the mouth is small. Pectoral and pelvic fins large, especially the former, which are somewhat ventrally placed. Two dorsal fins, the anterior over the pectorals, with a stout spine in front; and a small anal fin. Dermal denticles restricted to the claspers, and to localised areas on the dorsal surface in young forms. Dental plates large and thick, including a single pair in the lower jaw and two pairs, vomerine and palatine teeth, above, which combine trenchant edges with well-marked grinding areas. Three genera are known.
Fig. 267.—Chimaera monstrosa (male). m, Mouth; n.p, frontal clasper; op, operculum.
In Chimaera (Fig. 267) the mouth and nostrils are ventral, posterior to a bluntly conical snout. Head surmounted in the males by a club-shaped appendage armed with a pad of recurved denticles, the frontal clasper; there is also an anterior clasper armed with similar denticles and retractile into a shallow glandular pouch in front of each pelvic fin, in addition to the ordinary clasper behind the fin. The caudal fin consists of nearly equal-sized dorsal and ventral lobes, between which the slightly up-tilted caudal axis is prolonged as a long tapering filament: hence the tail appears to be nearly diphycercal. C. monstrosa occurs off the coasts of Europe from Norway to Portugal, including the Mediterranean, and also in the neighbourhood of the Azores, as far south as the Cape of Good Hope, and eastwards off the coast of Japan. It is the largest of the living species, reaching a length of 3 feet. C. affinis was first taken off the coast of Portugal, and subsequently on the North American side of the Atlantic, at depths ranging from 200 to 1200 fathoms. C. (Hydrolagus) colliei is restricted to the North Pacific, and is especially plentiful off South-eastern Alaska, and about the wharves at Esquimalt. Unlike most other Chimaeroids this species swims at the surface, and there is no evidence that it is a deep-sea form. In its breeding habits, and in the mode in which its eggs are fertilised, Chimaera probably resembles the oviparous Sharks and Dog-Fishes.
Fig. 268.—Egg-case of a species of Chimaera. a, Transverse section across the case at x, showing the lateral valvular slits; b, similar section across x′, showing the vertical ridges. (From Günther.)
