252 FISHES 



have been discovered in beds of this age. Climatius is an in- 

 teresting exception. This fish occurs in the rocks of Forfar- 

 shire. It belongs to the group of extinct Elasmobranchs called 

 Acanthodians, from the fact that there is a strong, straight 

 spine in the anterior margin of each fin, paired and unpaired. 

 There is a peculiarity in this fossil which occurs in no other 

 fish, namely, that instead of two pairs of lateral fins, there 

 are on each side of the ventral region a series of fin-spines, ap- 

 parently five in number, and this case affords the strongest 

 support to the theory that the original condition was a pair of 

 continuous lateral fin-folds, of which the pectoral and pelvic 

 fins are the remaining portions. 



It may appear that the occurrence of Climatius, an Elas- 

 mobranch fish, in the Lower Old Red Sandstone is inconsistent 

 with the conclusion drawn above, that the other fishes found 

 in this formation were inhabitants of fresh water, and with the 

 general view of fish-evolution adopted by the present writer. 

 But geologists tell us that the Lower Old Red shows a tran- 

 sition from marine to lacustrine conditions, and on the other 

 hand the Acanthodians themselves may possibly have been 

 fresh-water or estuarine fishes. They show in some of their 

 characters approximations to the condition of the earliest 

 Ganoids, namely the terminal mouth, flattened and enlarged 

 dermal plates with enamelled surfaces, and the partial develop- 

 ment of a gill-cover. We have no evidence, however, that the 

 Acanthodians were actually ancestral to the bony fishes. 



In the Carboniferous and Permian strata some remarkable 

 forms of the shark type have been obtained in recent years in 

 a very well-preserved condition. The most primitive and 

 earliest of these is Cladoselache, found in Lower Carboniferous 

 strata in Ohio. Its mouth is at the end of the snout. It is 

 from two to six feet long. The paired fins are attached hori- 

 zontally by a wide base, and have the appearance of enlarged 

 portions of an originally continuous fold. The tail is short and 

 high vertically, with an extremely heterocercal structure ; that 

 is to say, the termination of the trunk is bent upwards almost 

 at right angles and the ventral fin-rays are very long. The 

 x^canthodians already mentioned extend from the Silurian to 

 the end of the Permian and therefore go back to an earlier 

 period than Cladoselache ; but Professor Bashford Dean con- 



