468 FISHES CHAP. 



fin-spines of ancient Holocephali, 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 l and Rhynchodus occur 

 in the Devonian of either Eussia or Germany, and in North 

 America, and Palaeomylus only in the Devonian of North 

 America. 



Fam. 2. Squalor aiidae. General shape of the body similar 

 to the existing Hdrriotta. 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. 2 



Fam. 3. Myriacanthidae. 3 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 Litho- 

 graphic 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, xxxiii. 1895, p. 1. 



2 Smith Woodward, Proc. Zool. Soc. 1886, p. 527 ; and 1887, p. 481. 



3 Id., Ann. Mag. Nat. Hist. iv. (6), 1889, p. 275. 



