84 GENESIS OF THE ARIETIDA. 
the equivalent or parallel characters are often differentials. Thus, the keel was 
varietal in the lower species of Caloceras as compared with Psiloceras, and 
became a differential in the higher forms. ‘The same held good for the quad- 
ragonal form of Vermiceras, and its arietian sutures. A very instructive com- 
parison may be made between the cretaceous angulatus-like forms of Hoplites, 
and their approximately exact morphological equivalents in Schlotheimia, and 
yet no one well acquainted with their development and genesis would hesitate 
to use the channelled abdomen, pile, and form in both genera as true differen- 
tials. These characteristics do not indicate affinity between these cretaceous 
forms and Schlotheimia. 
Vaeck, in his article upon the hollow keel of the Falciferi,! makes somewhat 
similar statements, and gives details showing the presence or absence of this 
peculiarity in different species of Harpoceras. Though not prepared to agree that 
these forms really belong to the same genus, it has been evident to us for some 
time that the hollowness of the keel was a characteristic which was homoplastic 
in several distinct series, and it is not a mark of genetic affinity with Oxynoticeras, 
unless accompanied by other characteristics showing that the descent of the 
species possessing it was probably traceable to Oxynoticeras. Unless the nealogic 
stages show traces of this ancestry, it is not in itself a differential characteristic 
sufficient to bind the forms possessing it into the same genus. 
The development of the keel, channels, and pile in Arnioceras shows that 
they were new modifications in this series, as they were also in Caloceras. The 
keel, after its appearance in varieties of Arn. miserabile, became of specific value in 
semicostalum, and remained thereafter constant. The straight pile and peculiar 
genicule were also first of varietal value in miserabile, and then approached spe- 
cific importance in semicostatum, and became constant in other species. The 
channels were variable in all the species in which they appeared, except one of 
the most highly specialized, Arn. ceras. We have not, however, seen many spe- 
cimens of this species, and it is not unlikely that this form may, upon further 
research, prove to be as variable as the more generalized species. 
1 Bemerk. u. d. héhlen Kiel d. Falcif., Jarhb. geol. Reichs., XXXVIL., 1888, p. 311. 
i Siete 
