VEETEBEATA 



23 



two families they are wanting. Among Diplacanthidse 

 the genus Climatius is remarkable in having a series of 

 spines on each side between the ventral and pectoral 

 fins, which indicate the position of the primitive lateral 

 fold, from which the paired fins are supposed to have been 

 derived. 



Diplacanthidse are Devonic, as are also Ischnacanthidse. 

 Acanthodidge belong to the Devonic and Carbonic. 



Fig. 6.— Pectoral and ventral fins of Cladoselache xi/g (from Dean). B, axonoats (meso- 

 and meta-pterygium) ; R, baseosts ; D, dermal portion of fin. 



In the IcHTHYOTOMi the notochord persists, and the cen- 

 tra are represented by segments; the neural spines, axo- 

 nosts, and baseosts are continuous, and the dermal rays are 

 actinotrichia (that is, hair-like and more numerous than 

 the baseosts). There are two families, the Pleuracanthidse 

 and the Cladodontidse, which differ in the form of the pec- 

 toral fin. In the former they are biserially pinnate (Fig. 5), 

 while in the latter they are uniserially pinnate (branches 

 — baseosts — on one side of the axis — axonosts — only) (Fig. 

 6.) The Pleuracanthidse have teeth with two principal equal 

 cusps, while the Cladodontidse have a principal median cusp, 



