Prof. Agassiz on Fossil Fishes. 323 



of the fish of the older formations ; all in them, on the con- 

 trary, recall to mind the fish of our present oceans, so that be- 

 fore having made a minute study of them, we should be inclined 

 to think that we had to do with recent species. Their frag- 

 ments are buried in a more or less indurated clay, which some- 

 times presents the hardness of calcareous rocks, while, in other 

 localities, it has remained perfectly soft. The greater number 

 of the fish have rotted in this fine clay, their bones have sepa- 

 rated, and the soft parts have been replaced by clay. Now, 

 since it is no longer Ganoids with cuirassed body covered with 

 interlocked bony scales, but Cycloids and Ctenoids Avith thin 

 fragile scales, their coating has not been sufficiently solid to 

 preserve the integi'ity of their form and outline. Their body 

 has become decomposed, their fins have become detached, their 

 scales disaggregated, and of the greater number only the cran- 

 ium has remained, preserved entire, ovs^ing to the soldering of 

 the osseous pieces composing it. If, instead of belonging to 

 fish, these skulls were derived from Mammalia or reptiles, it 

 is to be presumed that all possible advantage would be taken 

 of them, and that the palaeontologist would have no trouble in. 

 determining their relations, since for these classes the mate- 

 rials for comparison are not wanting, the points of departure 

 are fixed. The characteristic features of the skulls of the Mam- 

 malia and Reptilia are known ; the variations which such a 

 bone, such a crest, such a groove may undergo in such and 

 such a family are understood ; and already, at the first glance, it 

 is possible to ascertain -whether the animal under consideration 

 is carnivorous, ruminant, or solipedal. 



But nothing is more variable than the forms of the cranium 

 and of the heads of fish. The multitude of bones and of spines 

 which serve for the attachment of the muscles, the infinite va- 

 riety of forms in the families themselves, imparts such a diver- 

 sity to the crania of fish, that the ichthyologist frequently 

 despairs of being able to reduce them to their respective types, 

 and in fact a comparative craniology of fish does not exist. 

 There is no one that I know who can tell at first sight whether 

 such and such a cranium belongs to a Percoid, to a Sparoid, or 

 to a Chetodontal type. 



The great majority of tlie fossils of Sheppey consists, we 



