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 pilaa, 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 Psil. planorbe, and its immediate congener, Psil. caliphyl- 

 him, 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 

 Europe 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. Even 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 Arietidoe 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, it is the smallest natural 

 group u'hich 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 Yermiceras, 



