DEVONIC FISHES OF THE NEW YORK FORMATIONS l8l 



of the tail, the paired fins arose similarly by the subdivision of continuous 

 membranes which extended as a symmetrical pair along the outside walls 

 of the body cavity. Even in the fashionable Acanthodians of the Silurian 

 and Lower Devonian periods (e. g. Cliraatius) the foremost and hindmost 

 pairs of spines were somewhat larger than the others ; and in all later mem- 

 bers of the group the intermediate "spines" dwindled to insignificance 

 (Mesacanthus*) or disappeared (Acanthodes), so that only the two normal 

 pairs of fins remained. 



Paired fins as paddles. It is, of course, possible that the fins of the 

 Acanthodians were stiffened by some kind of internal cartilage which was 

 never hard enough for fossilization ; but this seems improbable, because 

 even the stout arch to which the pectoral fins are fixed is shown by micro- 

 scopical examination to have been entirely a skin structure, and does not 

 appear to have ensheathed any internal cartilage. There must, however, 

 have been some primitive allies of the Acanthodians with their pairs of fins 

 reduced to the normal two, in which the stiffening was attained by internal 

 rods of cartilage instead of mere skin structures ; for a long-bodied (and 

 thus senile) survivor of this allied tribe occurs in the Upper Devonian of 

 Ohio [Cladoselache, text fig. 14, P- 55]- Here the fin flaps are strengthened 

 inside by a row of simple parallel bars of cartilage, which exhibit a tendency 

 to be squeezed together. 



The early fishes which had reached this stage were prepared for further 

 advance. Those which failed to make any progress in their skin skeleton 

 experienced very slight changes in their whole anatomy, and gradually 

 passed into the modern sharks and skates. Those in which the skin skele- 

 ton always remained extensive, and soon took the form of symmetrically 

 arranged bony plates and scales, rapidly became developed into the higher 

 fishes which swarm today. 



Fin flaps stiffened by internal rods of cartilage are essentially paddles, 

 and could be used for crawling in the mud as well as for ordinary swimming 

 in water. It is therefore interesting to observe that during the Middle and 

 Upper Devonian periods, when four-legged lung breathers must have been 

 just beginning to appear on the land, all the highest fishes had their fins in 

 the shape of paddles. It seems as if at that time there was a general 

 tendency for the fashionable and most advanced fishes to become crawlers 

 rather than swimmers ; and there can not be much doubt that the known 

 Crossopterygii, or " fringe-finned ganoids," as these fishes are commonly 

 termed, are the unsuccessful survivors of the race which originally produced 

 the earliest crawling lung breathers or Labyrinthodonts. . . It is one of 

 the problems of paleontology to determine the exact relationships between 

 the paddle-finned fishes and the lung breathers by the discovery of perhaps 

 Lower Devonian links. 



As fishes, the Crossopterygii were obviously not a success, for they 



