The Class Elasmobranchii or Shark-like Fishes 517 



and the ventrals move forward. In the Permian species the 

 pectorals are enormous. 



Traquairia pygmcea, from the Permian of Bohemia, is a di- 

 minutive sharklet three or four inches long with large scales, 

 slender spines, and apparently no ventral fins. 



In the genus Cheiracanthus the dorsal fin is placed before the 

 anal. In Acanthodopsis the teeth are few, large, and triangular, 

 and the fin-spines relatively large. 



The Ischnacanthidce have no clavicles, and two dorsal fins. 

 Ischnacanthus gracilis of the Devonian has a few large conical 

 teeth with small cusps between them. 



The Diplacanthidce, with two dorsal fins, possess bones 

 interpreted as clavicles. The teeth are minute or absent. In 

 Diplacanthus striatus and Diplacanthus longispinus of the Lower 



FIG. 304. Diplacanthus crassissimus Duff. Devonian. Family Diplacanthidos. 

 (After Nicholson). (Restoration of jaws and gill-openings; after Traquair.) 



Devonian stout spines are attached to the shoulder-girdle 

 between the pectoral spines below. 



In the very small sharks called Climatius the fin-spines are 

 very strong, and a series of several free spines occurs, as above 

 stated, on each side between the pectoral and ventral fins, a 

 supposed trace of a former lateral fold. In Paraxus the first 

 dorsal spine is enormously enlarged in size, the other spines 

 remaining much as in Climatius. 



Dean on Acanthodei. In his latest treatise on these fishes, 

 "The Devonian Lamprey," Dr. Dean unites the Pleuropterygii 

 and Acanthodei in a single order under the former name, re- 



