FOURTH, OR CORONICERAN BRANCH. 161 
species does not appear to indicate the exact affinities. Ver. (Ariet.) Conybeari, 
Plate XX. Fig. 6, Ver. (Ariet.) doricus, Figs. 8-10, and abjectus, Fig. 11, all seem 
to be the young of some species of Vermiceras, in which keel and pile are 
developed early, possibly some form of Conybeari. 
LEVIS STOCK. 
The living chambers may be cylindrical, or they may be broadened out and 
considerably modified by the growth of the whorl, but the length is invariably 
under one volution, and often does not exceed one half of a volution. There 
seems to be no necessary correlation between the shape of the chamber and its 
length; the most attenuated and cylindrical whorls have not invariably the 
longest living chambers, nor the broadest whorls invariably the shortest living 
chambers. It is however true, in a very general way, that the longest living 
chambers, as a rule, occur in the Plicatus Stock among the species which have 
the most attenuated or cylindrical whorls. 
None of the shells of the Levis Stock are directly or indirectly traceable 
to any known form of Psi. planorbe, var. plicatum. Their gradations and radical 
forms, nevertheless, do indicate derivation from planorbe, var. leve, and we must 
therefore consider the keels, channels, pila, and sutures which are similar in 
the two stocks as having originated independently in each stock, or, in other 
words, as homoplastic, and not homogenous characters. 
The higher species of each series tend to become involute, and to elongate 
the abdomino-dorsal diameter of the whorl. 
The sutures are almost purely arietian in proportions and outlines, the 
auxiliaries are rarely or never inclined posteriorly, the marginal digitations are 
less complicated, and the’ saddles broader and less dendritic than in the Plicatus 
Stock. 
FOURTH, OR CORONICERAN BRANCH. 
The shells are discoidal, and involution is limited to the area of the abdo- 
men. During senile degeneration the shell is apt to acquire flattened smooth 
sides and a narrow abdomen, but never loses the keel, nor becomes rounded on 
the abdomen, nor decreases in the amount of involution. Flattened sides and 
narrow abdomens may also appear in the adults of species with accelerated 
development of progressive characters. 
ARNIOCERAS. 
The members of this genus may be recognized by the smoothness and thin, 
discoidal psiloceran form of the first three or four whorls. The keel appears as 
an angular ridge, which develops later into a true keel. There are lateral folds 
in the young, which develop later into pile. The true pile appear after the 
keel, and then in some specimens well defined channels arise. 
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