148 



SUB-CLASS ELASMOBRANCHII. 



FIG. 87. Pleuracanthus decheni, 

 restored by A. Fritsch, x i ; 

 Lower Permian, Bohemia (from 

 Woodward). A 1 , A' 2 anal fins; 

 C dorsal part of caudal fin ; Z> 

 dorsal fin. The specimen from 

 which this figure was taken has 

 been crushed in such a way 

 that the paired fins appear to 

 have their postaxial sides 

 turned forward. 



into three pieces, and are twice as numer- 

 ous as the neural arches, those of the 

 dorsal part of the caudal fin are similar, 

 but equal in number to the neural arches ; 

 the anal fin is double, and its supports 

 are partly fused, and branch peripherally. 

 Didymodus Cope, Permian of Texas, skull 

 shows symmetrical fissuring, to which the 

 name of the sub-order is due. 



Order 4. SELACHII (PLAGIOSTOMI), 



Elasmobranchii with characters of 

 the soft parts as defined for the sub- 

 class, with hyostylic skull (except 

 Notidanidae) and heterocercal tail. 

 The notochordal sheath is always 

 segmented, though sometimes imper- 

 fectly ; the pectoral fins with three 

 basal cartilages, and the pelvic fins of 

 the male with claspers. 



The mouth is placed on the under 

 surface of the head, except in 

 Chlamydoselachus in which it is sub- 

 terminal, and Rhinodon and Rhina 

 in which it is terminal. The skin 

 has detached placoid scales only. 

 The body is either fusiform or flat- 

 tened dorsoventrally, and there is 

 usually a spiracle, but the pseu- 

 dobranch is absent in the Scym- 

 nidae, Lamnidae, Myliobatis, Trygon, 

 etc. In Carcharias and Zygaena, in 

 which the spiracles are absent, a 

 pseudobranch is present buried in 

 the flesh or placed on the front wall 

 of a recess of the mouth. They are 

 almost all marine, but a few ascend 

 American and Asiatic rivers, and a 

 few are confined to freshwater (some 

 Trygons, two species of Carcharias). 

 They have existed since Palaeozoic 

 times 



