DEVONIAN FISHES OF IOWA 61 



Even in the fashionable Acanthodians of the Silurian and 

 Lower Devonian periods ( Climatius, Fig. 5A), the foremost and 

 hindmost pairs of spines were somewhat larger than the others ; 

 and in all later members of the group the "intermediate spines" 

 dwindled to insignificance (Mes acanthus, Fig. 5B), or disap- 

 peared (Acanthodes, Fig. 50, 5D), so that only the two normal 

 pairs of fins remained. The fixation and stiffening of these fins, 

 however, were so completely unsuited for further elaboration 

 while they depended solely on skin-structures, that the Acantho- 

 dian fishes gradually declined towards insignificance and extinc- 

 tion. They lost their graceful fusiform proportions; some un- 

 wieldy and overgrown species became round-bodied grovellers 

 in the mud of Carboniferous seas and estuaries (Gyr acanthus) 

 while the latest members of the race, which did not increase much 

 in size, became almost eel-shaped before they died out in the 

 Permian period (Acanthodes, Fig. 5D). All races which do not 

 progress tend to become represented by eel-shaped species in 

 their latter days, and the Acanthodians formed no exception to 

 the rule. . . . 



Fig. 6. 



Fig. 6. Cladoselache fyleri Newberry. Cleveland shale (Upper Devonian); Cleveland, 

 Ohio. Right side-view, about one-tenth natural size. A primitive shark illustrating the 

 simplest kind of paddle-flns, which are supported by nearly parallel bars of internal car- 

 tilage (after Bashford Dean) . 



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 carti- 

 lage 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, Fig. 6). 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-skeleton 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. ' ' 



