GANOID FISHES. 307 



to this period which we cannot name here, but most of them be- 

 long to the two great classes designated by M. Agassiz, the Pla- 



c,oid, or plated; and the Ganoid, or shining, or enameled. The 

 first class, is at present represented by the sharks and rays, and 

 the second by the sturgeon and bony pike. The former or Pla- 

 coid, were first introduced, but wre comparatively small and 

 feeble, but the latter, during the Devonian, or old red sandstone 

 period, (see page 190), were very abundant. It is somewhat re- 

 markable that this group of fishes is now represented by only two 

 species, the American Gar-pike and the Birchir of the Nile; no 

 less than sixty distinct species being found in the old red sand- 

 stone. No traces however, of those groups of fishes, now so 

 abundant, are found in these strata. Leaving this remarkable 

 period, when the waters teemed with formidable and singular 

 shaped fishes, we find the commencement of a new epoch in the 

 appearance of an immense deposit of carbonate of lime, in the 

 form of the carboniferous or mountain limestone, and the varie- 

 gated marbles; imbedding a remarkable number of vegetable fos- 

 sils, which indicate not only the presence of land, but the exis- 

 tence of a luxuriant and tropical vegetation. The carboniferous 

 limestone, is in many cases the result of the labors of the coral 

 insect, indicating the presence of a shallow sea- bottom and warm 

 temperature. Immediately in contact with the coraline lime- 

 stones is a coarse sandy conglomerate used for millstones, and 

 called the mill-stone grit, which is succeeded by strata of shale 

 and sandstone, with occasional seams of coal; after which, follow 

 the coal measures, so called, being large deposits of bituminous 

 coal. But little doubt can exist as to the vegetable origin of di 

 coal; it is true that the most perfect bituminous coal has under- 



