———— 
=, 
bo 
GENESIS OF THE ARIETIDZ. 
Cal. Johnstom was at. first smooth, then ribbed, and the ribs had a peculiar 
fold-like character, and they appeared in this succession in the young of all the 
remaining series. The keel was added to these in the adult of Cad. tortile after 
the pile were developed. The adult of Cal. daquewm had the keel, and added 
also faint channels and in one variety the tubercles.’ If we are right in referring 
Cal. Deffneri® to this series because of its sutures, then the terminal species of 
Caloceras had a very highly accelerated development, producing the quadrago- 
nal form, keel, channels, complete pile, and tubercles at nearly the same stage 
of growth as in Ver. ophioides. 
In the development of the young of Ver. Conybcari, the succession of charac- 
ters was similar, — first a smooth whorl, then fold-like psiloceran pile, then keel, 
then channels, then true pile, which often became tuberculated. In Ver. ophioides, 
a tuberculated species with highly accelerated development, it is difficult to 
determine whether the keel was developed before or after the channels, since 
they appeared almost simultaneously. 
In the varieties of Cal. laqueum, we found the forerunners of all the rounded, 
quadragonal, keeled, channelled, and tuberculated forms of Vermiceras. The 
variety which led into Ver. spiratissimum had no tubercles, and could be called 
quadragonal, though it had only a slight keel and no channels, or at most 
very faint bands of depression on either side of the keel. In Ver. spiratissimum, 
these characteristics are fixed within narrower limits of variation, the keel, chan- 
nels, and quadragonal form were invariably present in adults, but better defined 
in some than in others, and no variety with tubercles has yet been discovered. 
In Ver. Conybeari also the characteristics above mentioned were more invariable, 
but here we found numerous adult individuals with tubercles on the pila, and 
varieties which produced all these advanced characteristics, including the tu- 
bercles in some cases, at a very much earlier age than in other species. Lastly, 
Ver. ophioides is always tuberculated, channelled, and keeled in adults, though, as 
shown by its young, evidently derived from those individuals of the tuberculated 
variety of Conybeart which did not produce these same characteristics at so 
early an age. 
In the arnioceran series, the keel appeared in the young before the channels, 
and also previous to the development of pile. This is the case in Arn. miserabile, 
and in the derivative Arn. semicostatum. This is in strict accord with an indepen- 
dent descent from the smooth Psiloceratites, but not with a supposed derivation 
from any intermediate caloceran or plicated form of Psiloceras. The pile ap- 
peared in the growth of an individual of Caloceras before the keel, — a condition 
due to the fact that the young remained, as previously explained, very similar to 
a plicated keelless Psiloceras during the earlier stages of development. The 
channels were first apparent in the adults of certain varieties of Arn. semicostatum, 
and in some other species of Arnioceras they became of specific value. 
1 This progression was much fuller in the species of the Mediterranean province (see Summ. PI. xi. 
fig. 17-19), showing the correlations of the development of the individual and evolution of forms in the 
series better than in Central Europe, 
+ Summ, Pl, x1. fig, 21. 
