56 GENESIS OF THE ARIETIDA. 
much obscured by morphological equivalents, and in such series the closest study 
of the structural gradations becomes the only sure guide. 
As a rule, a series also runs through a gamut from the discoidal forms to the 
involute, but not always, because there are series like Caloceras having no involute 
forms,and no one species of Vermiceras or Arnioceras is more involute than another. 
Nevertheless there is a decided development of the quadragonal whorl in Vermiceras, 
which, as shown in the series of species in Coroniceras and Asteroceras, and during 
the development of the individual in all normal forms of the Arietide, is usually 
an intermediate stage to the future genesis of compressed and involute shells. 
In such a system, also, certain radical forms which do not show the usual 
morphogenetic cycle may occur, as was the case with Psiloceras before the more 
involute forms of that genus’ were discovered in the Mediterranean province. 
These may have a closely allied and inseparable series of varieties,” which 
cannot be distributed into the different genera arising from them. In such 
cases, the radical may be considered as an undeveloped series, and separated 
as a distinct genus, though it consist of but one species with well marked varieties. 
A species is a definite step, or gradation in the morphogenetic cycle of the 
genus, and is distinguished by its form, amount of involution, sutural and other 
adult and senile characters, and the more or less accelerated development of 
the nealogic stages. In the descriptions, it will be noticed that the ephebolic 
characters of the ancestral form, though it may be a closely allied species, are 
nevertheless often accelerated in the nealogic stages, and the ephebolic stages 
then acquire some peculiar distinctive differentials. The aberrant pathological 
forms, and dwarfs of the same species, may often have more accelerated devel- 
opment than the normal forms, and sometimes simulate distinct species. These, 
as well as the normal varieties of species, have connections with other species 
which can only be properly estimated with sufficient materials and accurate 
study. After having secured the genealogy of a series, the species can be deter- 
mined and separated, but until this is done, the work does not rest upon a secure 
basis. The possession of a keel, or channels, or a line of tubercles, or increased 
involution in the adult whorls, may distinguish one species from another in the 
same series; but the same differences may make the shell appear to be identical 
with a species occurring in another genus, and thus confuse the classification 
unless the genesis of the characteristics has been traced. 
The order adopted for illustrating the series in the Summary Plates is the 
result of following out genetic lines, and therefore presents forms in their ap- 
proximately natural relations, though necessarily having no reference to chronol- 
ogy. The species are connected by lines indicating their natural affinities, and 
show the relations of the series; but the title, Summary Plates, fully explains 
the necessarily abbreviated and more or less artificial nature of the arrange- 
ment. Comparison with Genealogical Table V. will serve to correct any erroneous 
impressions which might arise from the study of these plates, in so far as the 
species of Western Europe are concerned. Those from other localities, also 
figured in the Summary Plates, will be found by reference to the descriptions. 
1 Summ. PI. xi. fig. 11-13. + Summ, by si, fio, 15/25 Pl; xi fie, 
i 
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