322 Pi'of. Agassiz on Fossil Fishes. 



for there is no known fish-bed which counts so many species, 

 and no collection of tertiary fossils of England which does not, 

 at least, contain some specimens. The want of knowledge 

 which has hitherto prevailed Avith respect to the fish of Shep- 

 pey is solely owing to the very peculiar difficulties which the 

 study of their fragments present. Elsewhere, and especially 

 in the strata of transition and secondary rocks, in the slates, 

 limestones, and sandstones, the ichthyolites are more or less 

 entire ; and it is seldom that a fragment does not present seve- 

 ral parts of the body, diff'erent portions of the fins, of the scaly 

 coating, of the opercular apparatus ; or, if indeed the pieces 

 themselves are not preserved, their impression indicates at 

 least the general form and the outlines of the body ; so that, 

 with a sufficient knowledge of living fish, of their form and 

 external characters, it is possible to arrive at accurate and 

 strict determinations. Moreover, the majority of older fish 

 have osseous scales harder even than the bones ; and the mode 

 of their arrangement (enchevc trement) contributes to preserve 

 the general form of the fish even when the bones have disap- 

 peared and the other parts have become destroyed. 



Classifications in ichthyology have hitherto been based on 

 external characters; among others, on the form, number, and 

 position of the fins, the structure of the scales, the relations of 

 the diff'erent parts of the body to each other, the dentition, 

 the arrangement of the opercular j)ieces, &c. - If we glance 

 over the most esteemed works of our time on the natural his- 

 tory of fish, none but external characters will be met with in 

 the diagnoses of the families, genera, and species, easily con- 

 ceived, and sufficient, indeed, for the proposed object. If I 

 speak of the voids which this branch of science, to which I 

 have devoted myself for so many years, presents, it is not that 

 I wish to detract the least in the world from the merit of so 

 many works which the most distant posterity will still regard 

 as master-works of sagacity, application, and research ; but it 

 is that, having selected a special branch of ichthyology, I have 

 perhaps been enabled more than others to perceive what re- 

 mains to be done in this vast domain. This is especially true 

 with respect to the Sheppey fish, Avhich have none of those 

 forms and of those fantastic characters peculiar to the majority 



