558 The American Naturalist. [June, 
diverging very much as compared with the smooth discoidal shell of 
Psil. planorbe, they were in reality parallel to each other, that is to say 
each went through with a similar cycle of changes ; the discoidal smooth 
whorl became quadragonal, and acquired keel, channels, and ribs in 
the species representing the acme of their progress, while after the 
acmatic period of the species there was a tendency to produce shell, hav- 
ing more involute compressed and acute whorls. Throughout all of 
these, however, in each series a few characteristics were acquired and 
transmitted by which each series might be distinguished from its allied 
series, 
Another result is that the whole of the group of the Arietidae arose 
and died out within the limits of the Lower Lias, and that there are 
three grand faunas, the earlier, the central, and the latest in time, these 
three agreeing in their general characteristics with the development 
and decline of the individual and with the cycle shown by each series. 
Thus the earliest faunas are everywhere composed in the mass of simple 
discoidal forms, the central of still discoidal shells, but these have keels, 
channels, and ribs ; the latest faunas are characterized by the prevalence 
of involute, compressed, and often smooth shells. The method of 
classification was the result of practical work during which the young, 
adolescent, adult, and old age stages of many of the species mentioned, 
and in most of these species all of their known varieties were studied, 
these observations were correlated in all directions with the observed 
difference and resemblance of the species. Thus the characteristics _ 
of the young and adolescent stages were compared with the adult char- 
acteristics of the ancestral forms in each series, and the characteristics 
of the old age of each form with those of the descendants in the same 
series whenever they exhibited any similar degradational characters, 
which was the case in nearly all series, After the series had been estab- 
lished by this process, succession and relation of the forms was com- 
pared with their actual succession in the rocks, and the results showed 
agreement in every series, except where a series, as sometimes happened, 
occurred altogether on the same level. In order that the evidence 
could be judged by the reader, lists of names of species and their level 
of occurrence have been given in six different tables from five different 
basins in Europe. It was found while following out this last investiga- 
tion that in some localities new forms had arisen, and that others had 
received their Ammonitic population wholly or in large part by migra- 
e former, which were called aldainic basins, are in strong con- 
trast with the analdainic or unproductive basins. It was found that 
the aldainic basins formed a band running westward, beginning in the 
