122 MANUAL Of THE MOLLUSCA. 
‘‘ Hehini are most common in the superior strata; 
** Ammonites to those beneath ; 
‘** Producti, 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 differ 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 :—Palzeozoic, Secondary, and 
Tertiary. 
Thirteen geological systems, each haying 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. t 
* The Pliocene 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 gn additional “‘ Formation,” much less a ‘‘ Quaternary System,” 
+ It was on this account that Professor Sedgwick proposed the term “ Palzozole, ? 
rather than “ Protozoic,” for the oldest fossiliferous rocks, 
