338 M. Agassiz on Fishes. 



It is in the series of deposits inferior to the Lias that we begin to 

 discover the largest of those monstrous Sauroid fishes, whose oste- 

 ology approximates them so much to the skeletons of the Saurians, 

 by the stronger sutures of the cranial bones, by their great conical 

 teeth with longitudinal striae, and also by the mode in which the 

 spinous apophyses are articulated with the bodies of the vertebrae, 

 and the ribs also at the extremity of the transverse apophyses. The 

 analogy between these fish and the Saurians goes further than the 

 skeleton : in one of the two species which is now alive, I have found 

 an interior organization of soft parts, which is very peculiar, and 

 which approximates these groups of animals more than at first was 

 anticipated. In the Lepiclusleus osseus there is a real glottis, like 

 that of the sirens and the salamander reptiles, also a cellular swim- 

 ming bladder, with a trachea, as in the lungs of an ophidian. Fi- 

 nally, these integuments have often an appearance so like to those of 

 the crocodile, that it is not always easy to distinguish them. 



The small number of fishes found in the transition rocks appear 

 not as yet to enable us to assign to them any peculiar character. At 

 the same time, the sj)ecies in the collection of iWr IMurchison exhibit 

 types which are not found even in the coal measures. 



What is very remarkable in all the fishes below the oolite species, 

 besides their resemblance to reptiles, is, on the one hand, the greatest 

 uniformity of their types ; and on the other, the very great uniformity 

 of the parts of the same animal among tliemselves ; so much so, that 

 the scales, the bones, and the teeth of one are with difficulty distin- 

 guished from those of the other. And if we might here hazard a 

 conjecture on this state of matters, we should be naturally led to 

 fancy that the principle of animal life, v.hich developes itself at a 

 later period under the form of our common fishes, of reptiles, birds, 

 and beasts, is, at first, entirely confined to these singular sauroid 

 fishes, at the same moment participating of the nature of fishes and 

 of reptiles ; and that this mixed character is only lost in this class 

 upon the appearance of a greater number of reptiles, as we see the 

 Ichthyosaurus and Plesiosaurus participate, as to their bony structure, 

 in the characters of the cetaceae, of the class of mammiferce; and the 

 great terrestrial saurians to those of the Pachydermse, which would 

 appear to have been called into existence at a much later period. 



We are thus by observation conducted to those ideas of the philo- 

 sophy of nature which are presented to us by an organic and regular 

 development of all created beings, in fixed relation to all the different 

 conditions of existence which are realized on the surface of the globe, 

 at the c(mclusion of those changes to which it has itself been sub- 

 jected. 



According to all these facts, I perceive in these series of all the 

 geological formations two grand divisions, which have their limits at 

 the greensand. The ^"ormer, and more ancient, includes only the 

 Ganoides and Placoides ; the latter, more closely allied with the 

 existing species, comprehends forms and organizations much more 

 diversified ; and these are principally the Ctenoides and the Cycloi- 

 des, and a very small number of the species of the two preceding 

 orders, which insensibly disappear, and the living resemblances of 

 which are very considerably modified. As I do not find in the fishes 

 of the first grand period the differences which correspond to those 



