k 



Embryonic Aye of the Reign of Fishes, 41 



so strikingly in the geological series : namely, that all the 

 fishes belonging to formations more ancient than that of the 

 Jura, have the extremity of the caudal raised, and the caudal 

 itself heterocerque. 



There is, lastly, another point to which I would solicit the 

 attention of naturalists ; that is, the form of the head and 

 position of the mouth and eyes in fishes of the old r(;d sand- 

 stone. All, without exception, have the head large and 

 flattened, rounded, and as it were truncated, similar to that 

 of a Lotta or Silurus. This character preponderates to such 

 a degree, that it is very rare to see a fish of the old red sand- 

 stone which presents the head in profile ; in the majority 

 of cases, it rests on the upper or lower surface, even when 

 the body is lying in such a manner as to present one of the 

 sides. The mouth of the greater part of the genera is widely 

 open, semicircular, placed either at the extremity of the 

 rounded head, or even under it. The eyes, in the majority 

 of the genera, are widely apart, and thrown to the flattened 

 sides of the head, in such a way that it is often very difiicult 

 to determine their position. Analogous forms are found in 

 embryos. Even among fishes which, at a later period, are dis- 

 tinguished by a long snout in the form of a beak, the embryos 

 have at first a broad rounded head, truncated in front, with 

 the mouth below, and the eyes lateral, and it is not till later 

 that the jaws become elongated and project before the eyes, 

 forming at last a head of an entirely different form from 

 what it exhibited at first. 



I believe that it would not be easy to find more numerous 

 approximations between the embryos of our fishes and fossil 

 fishes, since no part of their bodies is preserved to us but 

 the osseous system which alone has furnished all these analo- 

 gies ; and I think observers will generally agree with me 

 when I affirm that the fishes of the old red sandstone represent, 

 in the whole of their particular structure, the embryonic age of 

 the reign of fishes. In no instance, in fact, in any other forma- 

 tion, do we find so great a number of fishes in which the 

 internal skeleton is so imperfectly developed, and so inferior 

 to the cutaneous system ; nowhere else do we find the great 



