4^13 MANL'AL OF THE MOLLUSCA. 



the larger groups have distinct sub-genera, genera, and families, according to 

 their size and importance. 



Wm. Smith himself observed that "Three principal families of organized 

 fossils occupy nearly three equal parts of Britain." 



" Echini are most common in the superior strata ; 

 "Ammonites to those beneath ; 

 " Produdi with numerous E,icrim to the lowest," 

 This kind of generalization has justly been considered, by Prof. E. Forbes, of 

 higher importance than the identification of strata by species ; a method only 

 applicable to moderate areas, and becoming less available with distance. 

 Indeed it might be assumed that strata geographically distant, yet containing 

 some identical species, must differ in age by the time required for the migra- 

 tion of those species from one locality to the other. 



A table of the characteristic s/^^-c/^,? of the English strata is of little use in 

 America or India, except to shew how few and doubtful are the identical 

 fossils. Whereas the characteristic genera, and order of succession of the 

 larger groups are the same at the most distant localities; and whatever value 

 there may be in the assumption that particular systems of rocks contain most 

 workable coal, lead, or rock-salt, is not lessened by the circumstance that the 

 species of fossils in those rocks are not everywhere the same, since the genera 

 alone are sufficient to identify them. 



Genera, like species, have a commencement, a climax, and a period of 

 decline ; the smallest usually range through several formations, and many of 

 the typical genera equal the families in duration. 



Groups of formations are called Systems, and these again are combined in 

 three principal series— Palaeozoic— Secondary— and Tertiaiy. 



Thirteen geological systems, each having a number of peculiar genera are 

 shewn in the accompanying table. (No. 11.) Some of the genera cited have 

 a wider range, like Belemniies. but are mentioned because of their abundance 

 in one particular system. The names in italics are existing genera.* 



The third table contains the names of some of the larger genera, arranged 

 according to the order of their appearance. This diagram conveys the 

 impression that the series of fossiliferous strata is not completely known ; or 

 that the beginning of many groups of fossils has been obliterated in the uni- 

 versal metamorphism of the oldest stratified rocks. f 



* The Pliocene strata contain no extinct genera, and represent only the com- 

 mencement of the present order of things. All the deposits now taking place will 

 not constitute an additional " Formation," much less a " Quaternary System." 



t It was on this account Prof. Sedgwick proposed the term " Palaeozoic, rather j 

 ^han " Protozoic," for the oldest fossiliferous rocks. 



