PLEURA CANTHODII 



183 



pectoral fin at this stage remains doubtful, but there is little 

 evidence of their existence in the pelvic fin (p. 181). The reduc- 

 tion of the dermal skeleton, and the development of a large occipital 

 spine in the first family, are signs of specialisation. Possibly 

 the Pleuracanthodii are related to the Cladoselachii through the 

 Cladodontidae (Dollo [121], etc.). The Pleuracanthodii appear 

 in the Devonian epoch, and seem to die out in the Permian. 



Family PLEURACANTHIDAE. Pleuracanthus, Ag. ; Lower Permian, 

 Europe. (Xenacanthus, Beyr. ; Carboniferous and Permian, Europe ; 

 possibly the same as Pleur acanthus.) Diplodus, Ag. ; Carboniferous and 



FIG. 148. 



Left pectoral girdle and fin-skeleton of Pleuracanthus Gaudryi, Fr. a, segmented axis of 

 lln ; c-, coracoid region ; d.r, dorsal cartilage ; pr, preaxial radial ; pt, postaxial radial ; 

 x, scapular region ; t'.o, ventral cartilage. On the right three teeth of Orthacanthus (from 

 Fritsch), t. 



Permian, N. America. Diacranodus, Garni. ; Permian, N. America 

 [61]. 



Family CLADODONTIDAE. With teeth having many small lateral cusps, 

 and uniserial pectoral-fin skeleton (Fig. 87). The rest of the skeleton is 

 scarcely known at all (Traquair [464], Braus [49]). 



Cladodus, Ag. ; Devonian, Carboniferous, and Permian of N. America, 

 and Europe. Symmorium, Cope, Carboniferous, N. America, possibly 

 belongs to this sub-class (Fig. 87) (Cope [92]). Dicentrodus, Traq. ; Mid. 

 Carboniferous, Scotland. Phaebodus, St. J. and W. ; Devonian, N. 

 America. 



Family CHOXDRENCHELYIDAE. An incompletely known fossil 

 described by Traquair [452a] ; it is like Pleuracanthus, but lacks the 

 dorsal spine, and has only one dorsal radial to each segment. 



Chondrenchelys, Traq. ; Lower Carboniferous, Dumfriesshire. 



