Essay mtroductory to Geology. 5 



formations. The genus terebratula also, which pervades all the'geological 

 formations and still continues to exist, has a few but more rare representa- 

 tives, even in this early period. 



Of bivalves less widely distinguished from recent types, a few cardia, 

 and perhaps oysters and pectens, are said to have been found, but these 

 are comparatively rare. 



Of the gasteropodous univalves we have a few very peculiar forms, appa- 

 rently congenerous with the pectinibranchia trochoidea. Some of the 

 buccinoidea are enumerated in lists, but these are certainly very rare. 



Of the concamerated shells, which contain cephalapodous inhabitants, 

 (a race to which the cuttle fish belongs, though destitute of a shell, in 

 place of which the mantle has only secreted the well known lamelliform 

 calcareous bone,) the lowest fossiliferous beds present several species of 

 the straight family, the orthoceratites, whicli has entirely ceased to exist, 

 and is represented under a very different form by the belemnites of the 

 secondary rocks. Of the spiral families, the ammonite, which continues 

 through the secondary formations, but had become extinct before the ter- 

 tiary, and the nautilus, which pervades the whole geological series, had 

 already appeared, but the species are distinct and the specimens rare. 



Still ascending in the animal kingdom, we find among the marine Crus- 

 tacea a remarkable family, the trilobite, or Dudley fossil, of which the 

 lowest fossiliferous beds afford three genera, although the whole family 

 appears to have become extinct before the period of the secondary 

 formations. 



With regard to the fossil vertebrated fish, their history is only at the 

 present moment beginning to be understood, through the labours of M. 

 Agassiz, of which only two livraisons have yet appeared. His remarks, 

 however, on this branch of our subject, are strongly confirmatory of those 

 which have been already suggested by other departments of the animal 

 kingdom. In his perfectly original method, he divides fish into four orders. 



1. Placoidians, including Cuvier's cartilagineae, with the exception of 

 the sturgeon. 



2. Ganoidians, fifty extinct genera, only plectognathus, syngnathus, and 

 acipenser recent. 



3. Ctenoidians, including acanthopteryglans, minus those with smooth 

 scales, and phis the pleuronectes. 



4. Cycloidians, the malacopterygians, minus the pleuronectes, and plus 

 the species banished from the last order. 



The two latter orders are prevalent in the existing state of things, com- 

 prehending more than three-fourths of the fishes which now live; but ia 

 the geological series, these orders are confined to the superior formations, 

 vix. the chalk and tertiaries ; and in all tlie lower formations, the two first 

 orders which now constitute merely a small minority, not only become the 

 tnajority, but prevail altogether exclusively. 



