122 MANUAL Oi? THE MOLLUSCA. 



** Echini are most common in the superior strata ; 

 " Ammonites to those beneath ; 

 ** Produdi, with numerous Encrini, to the lowest." 

 This kind of generalisation has justly been considered by Pro- 

 fessor 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 difier in age by the time required for the 

 migration of those species from one locality to the other. 



A table of the characteristic species of the English strata is of 

 little use in America or India, except to show 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 con- 

 tain 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 

 Tertiary. 



Thirteen geological systems, each having a number of peculiar 

 genera, are shown in the accompanying table. (No. II.) Some 

 of the genera cited have a wider range, like Belemnites, 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 fossili- 

 ferous strata is not completely known ; or that the beginning of 

 many groups of fossils has been obliterated in the universal 

 metamorphism of the oldest stratified rocks. f 



* The Miocene strata contain no extinct genera, and represent only the commence- 

 ment of the present order of things. All the deposits now taking place will not consti- 

 tute s.n additional " Formation," much less a "Quaternary System." 



t It wiis on this account that Professor Sedgwick proposed the term " Palaeozoic," 

 rather than " Protozoic," for the oldest fossiliferous rocks. 



