230 



EXPLANATIONS. 



he regards the history of the class as so far from complete, 

 that the a umber of species successively entombed in the 

 crust of the globe might be estimated at thirty thousand, 

 without any chance of approaching the truth 1 * If such 

 be the case, we may surely expect to hear of other fishes 

 prior to or contemporary with the cestraceon, showing 

 that, humble as that animal was, it is not to be regarded 

 as the initial of its class. f But even although simpler 

 fishes be not found in lower or contemporary strata, this 

 may only be owing, like the non- discovery of vegetation 

 in the early rocks, to the unsuitableness of these fishes 

 for being preserved. Supposing the inferior tribes, petro- 

 myzonidae (lampreys) to have been then in existence, we 

 should have no trace of them preserved, because of their 

 osteological structure being slight, and their wanting 

 those teeth and spines which form, after all, the chief 

 memorials of the higher families of their own order. 



One word more as to these fishes. The critic says (p. 

 33,) it is shown to demonstration in the Poissons Fossiles 

 of Agassiz, that " the sauroids, in their general osseous 

 structure, and in the development of their nobler organs, 

 run close upon the class of reptiles." There is no doubt 

 that the sauroid fishes partake of reptilian characters, 

 though, perhaps, in a more external and less important 

 way than such writers as the Edinburgh reviewer sup- 

 pose; but, be it remembered, the sauroids are not the first 

 fishes. There is not one of them In the Silurian forma 

 tion, where placoideans appear to begin. Yet I do not, 

 for this reason, suppose that the sauroids arose from pla- 

 coideans. More probably, they are part of a distinct line 

 of development, which had inferior forms in its first 

 stages, also of too slight a structure to be preserved. 



Following this reviewer into his discussion of the Car- 

 boniferous System, we find him commencing with a 

 taunt, that there are now traces of land vegetation in 



* Review of Professor Pictet's Traite Elementaire de Palreonto 

 logie, translated in Jameson's Journal from the Bibliotheque Uni- 

 verselle de Geneve, No. 112, 18-15. 



t Such shifts are of frequent occurrence in geology. Insects, 

 formerly found first in the oolitic formation, are now taken back 

 So the carboniferous. Birds are now inferred from foot-tracks in 

 the New Red Sandstone, their first place formerly being in the 

 oolite. We have mammifers in the oolite, which, a few years ago, 

 were believed not to occur before the tertiary. None of these 

 shifts, however, in the least interfere with the general fact of the 

 advance from the lower to the higher classes of animals. 



