M. Agassiz on Fishes. 



337 



in a general classification of geological formations, it would appear to 

 me more natural to associate the chalk and greensand series with 

 the tertiary formations, than to rank them with the secondary rocks. 

 Underneath the chalk there is no longer a single genus which possesses 

 existing species ; and even those of the chalk which have them, pos- 

 sess a much greater number of fossil ones. 



The oolite group to the lias, also included, forms a very natural 

 and very well defined series, which must also comprehend the Weal- 

 den rocks, in which I have not found a single species referable even 

 to the chalk genera. From that epoch, always descending, the two 

 orders which prevail in the present creation are no longer founds 

 whilst those which are in small numbers in our days suddenly present 

 themselves in great abundance. As to the Ganoides, the genera with 

 the symmetrical caudal fin are those which are here found ; and 

 among the Placoides, it is especially those with teeth furrowed on 

 their two faces, and with large spinous rays, which predominate. 

 For it is now certain that those great rays which Dr Buckland and 

 M. de la Beche have denominated Ichthyodorulithes belong neither 

 to the Silures nor to the Balistes, but are the rays of the dorsal fin 

 of the great Squali, the teeth of which are found in the same beds. 



In quitting the Lias, and proceeding to the lower formations, a 

 great difference is observed in the form of the posterior extremity of 

 the body of the Ganoides. All of them have the vertebral column 

 at its extremity prolonged into an imperfect lobe, which reaches to 

 the extremity of the caudal fin, and this peculiarity extends to the 

 most ancient fishes. Another observation, which is worthy of re- 

 mark, is, that down to the coal formation, no fishes that are evident- 

 ly carnivorous are found; that is to say, with large conical and sharp 

 teeth. The others appear to have been omnivorous, their teeth being 

 rounded, or obtusely conical, or pencil-shaped. 



The day is assuredly not far distant when we shall be able 

 to collect a great number of facts respecting the habits of these ani- 

 mals, and their interior organization. The discovery of coprolites 

 already enables us to discover the organized beings which preyed upon 

 the pirates of the ocean ; for in these coprolites, which are suffi- 

 ciently numerous in those deposits which contain Sauroid fishes, we 

 easily discover the scales of the fishes which they ate, and sometimes 

 we can determine the scales. Their intestines even are preserved in 

 some cases, as for example in a specimen of Megalichthys, where a 

 portion of intestine is visible ; and bundles of pyloric appendices, and 

 the cul-de-sacs of intestines of the species Leptolepis and Thrissops 

 of Solenhofen, known under the name of Lumbricaria, are not rare 

 in the schists of this interesting locality. In the fishes of the 

 chalk group, there may be seen, in Mr Mantell s collection, speci~ 

 mens of Macropoma, in which the whole stomach is preserved with 

 its different membranes, which are separated, as it were, into as many 

 leaves. In a great number of the fishes of Sheppey, of the chalk and 

 oolite series, the capsule of the eye-ball is still preserved ; and in 

 many species of Monte-Bolca, of Solenhofen, and of Lias, there are 

 very distinctly to be seen all the little laminae which form the gills. 

 It also appears evident that the constitution of some rocks is much 

 more favourable for the preservation of animal remains than that of 

 others. 



