468 FISHES CHAP. 
fin-spines of ancient Holocephal, dental plates, closely resembling 
those of modern Chimaeroids and referred to the Ptychodontidae, 
are probably the earliest indications of the existence of the 
group. The Holocephali become more abundant in the Mesozoic 
period, but of the four families usually recognised, only one, the 
Chimaeridae, has survived. 
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' 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 Hurriotta. 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.” 
Fam. 3. Myriacanthidae.’— 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) ‘Shas its rostrum terminating in a 
cutaneous flap, as in Callorhynchus. Myriacanthus, from the 
Lower Lias of Lyme Regis, and Chimaeropsis, from the Litho- 
eraphic 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 
1 Rohon, Verhandl. k. Min. Ges. Petersburg, xxxili. 1895, p. 1. 
* Smith Woodward, Proc. Zool. Soc. 1886, p. 527 ; and 1887, p. 481. 
3 Id., Ann. Mag. Nat. Hist. iv. (6), 1889, p. 275. 
