GENEALOGY. 55 
arietian type in the form and sutures of Arnioceras, and from this the typical 
genus of the family, Coroniceras, was evolved. The keel, double-channelled 
abdomen, straight geniculated pile, and less complicated sutures of both genera, 
are similar to those of Vermiceras in the Plicatus Stock. 
This normal type was rapidly departed from in the next branch, in which 
the highly aberrant compressed Ast. Collenoti appeared. This aberrant ten- 
dency was still more decidedly brought out in the more rapid production of 
similar, but more compressed and involute, forms in Oxynoticeras. In this, also, 
the highest specialization was reached by the introduction of a new structure, 
the hollow keel, as a nealogic and ephebolic characteristic. 
The smooth variety of Psi/. planorbe, and its immediate congener, Psil. caliphyt- 
um, were of course the most primitive forms which occurred in the Lias, and we 
can treat the whole of the two stocks as a connected group arising in Central 
Kurope from the smooth variety of planorbe, though, as a matter of fact, this is 
probably artificial. The actual process of the evolution of the second branch, and 
probably of Caloceras, as will be explained in Chapter III., took place in the 
basin of the Northeastern Alps, and the forms found in Central Europe were 
migrants. When arranged naturally the genera appear as in the Summary 
Plates, as an assemblage of distinct and more or less divergent series. 
We have considered each separate genetic series as a genus, because it was 
necessary to do this, or else use a cumbersome trinomial or quadrinomial descrip- 
tive nomenclature. Kvyen with the aid of binomials, we have not been able to 
speak of any series under one name as a single species. Had this rule been 
adopted, i.e. to treat each series as a single species, the opinions of paleontolo- 
gists are not now in its favor, and probably no one would have followed us in 
practice, however much disposed theoretically to praise our conservatism. Even 
Quenstedt in his most recent work has proposed names for the different groups 
of Arietide all ending in “ceras.” They are highly appropriate euphonically, 
but for the most part are open violations of the law of priority in nomenclature 
and not systematic in arrangement, though supported by observations and a 
wealth of accurate illustrations which are of the highest importance to all stu- 
dents of this branch of science. 
We have tried to show, in the Introduction and in other parts of this essay, 
that the metamorphoses of a normal individual in all its stages is a trustworthy 
index of the morphogenesis of its group, and that a group of species tended to 
have a cycle of forms corresponding to these metamorphoses. The unit of classifi- 
cation is, therefore, not the species, but the genus; in other words, ut is the smallest natural 
group which is genetically connected, and in which a more or less complete cycle of 
forms or species may be traced. The genus may also be further defined as an 
independent group of species, which must always be represented by a distinct 
diverging line when represented graphically in a geological diagram or genea- 
logical table. In such examples the genus becomes a series of forms, having a 
distinct line of modifications traceable to the adult radicals, and more or less 
present in the nealogic stages of their descendants. It has differential charac- 
ters, but these may be, as in the case of Coroniceras with relation to Vermiceras, 
