40 Heterocercal Tail of Old Bed Sandstone Fishes. 



from a single fin running along the hinder part of the body, 

 nearly like the fin of an eel. This continuous fin undergoes 

 a complete transformation in certain places ; in others, it 

 disappears little by little ; and where it remains stationary, 

 the rays are gradually enlarged. The spaces which sepa- 

 rate the different fins are, therefore, smaller, and so much 

 the less strongly marked the younger the embryo is. To 

 such a degree is this the case, that certain fishes which at a 

 later period would possess very distinct fins, have them very 

 close to each other at an early age, and sometimes scarcely 

 separated by a shallow notch. In the fishes of the old red 

 sandstone, the vertical fins enter completely into these primi- 

 tive conditions of development. The whole of the important 

 family of Sauroides, which at a later period appear provided 

 with well separated and insulated fins, is represented in the 

 old red sandstone only by the Dipterians, which are all pro- 

 vided with two anals and two dorsals, very near each other, 

 and but a short way from the caudal. In the Celacanthes of 

 the old red sandstone, we likewise find many genera, as the 

 Glyptolepis, and probably also the Platygnathes, which had 

 double vertical fins, and so closely placed that there was 

 scarcely an intermediate space between them. Even among 

 the Acanthodians there is one genus, that of the Diplacanthes, 

 which is furnished with double vertical fins. It is true that 

 this arrangement does not occur in all the genera, but it is 

 at the same time curious that the families which are destined 

 to run through a long series of formations, such as the 

 Sauroides and Celacanthes, commence with forms having 

 double fins, thus approaching the embryonic type. 



The fact, that among all the fishes of the old red sandstone 

 which possess a caudal, that fin is composed of unequal lobes, 

 and inserted on an elevated extremity of the dorsal cord, is 

 another point of approximation to the embryo of ordinary 

 fishes. We know that, in the latter, the extremity of the 

 tail begins to rise upwards at a certain period of life, ap- 

 proaching in this to the disposition observed in the sturgeon, 

 and that at this epoch the caudal of the embryo is hetero- 

 cerque. On the other hand, I have often called the attention 

 of naturalists to a fact in every respect similar, which appears 



