FOSSIL VERTEBRATES — FISHES 403 



of most of the sharks are in this form enlarged into scalelike 

 forms that fit tightly one against the other and afford a com- 

 plete cover for the body. The paired fins are more- strongly 

 developed than in the previous form, and are better fitted for 

 the purpose of balancing the body as well as assuming, to some 

 slight extent, the function of locomotion. 



Climatius, from Devonian Old Red Sandstone of Scotland, is 

 of considerable interest from the fact that between the paired 

 fins on the two sides there are developed many smaller fins, 

 located along the line of the primitive fin fold. These are 

 regarded as the remnants of the disappearing lateral body fold. 

 The form represents a stage in this respect ancestral to both 

 Cladoselache and Acanthodes, but is in other respects in advance 

 of both of them. 



PletiracciJitlins, from the Permian, is one of the most interest- 

 ing of the fossil sharks. It represents a stage considerably in 

 advance of the forms already described. The radials of the 

 median fins have separated off the proximal basal segments 

 that afford the strong attachment of the fin to the body wall, 

 and there is developed to some extent the dermal elements of 

 the external edge of the fin that are found in the fins of the 

 modern bony fishes. The paired fins present a very interesting 

 condition, the fore limb having the character of a dipnoan fin, 

 and the hind limb the characters of the more advanced type of 

 the fish fin. To understand this condition it is necessary to go 

 back to the formation of the paired fins from the lateral fin 

 folds. The gradual development of the functional swimming 

 fin was by the concresence of the basal and the radials to form 

 strong, though somewhat flexible supports for the membrane of 

 the fin. This was accomplished in two ways. In one the basals 

 united and formed a long median axis upon each side of which 

 the radials were attached after the manner in which the barbs 

 of a feather are attached on each side of the quill. This type 

 was originally supposed to be the most primitive form of the 

 fin, and so it was given the name of archipterygiiwi. In the 

 other type the basals fused into one or more pieces that were 



