84 GENESIS OF THE AKIETID.E. 



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, pila?, 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, 1 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 pike 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 Am. miserable, became of specific value in 

 semicostatum, and remained thereafter constant: The straight pilae and peculiar 

 genicular were also first of varietal value in miserabUe, 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, Am. ccras. 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. hohleu Kiel d. Falcif., Jarlib. geol. Reichs., XXXVII., 18SS, p. 311. 



