FOURTH, OR CORONICERAN BRANCH. 161 



species does not appear to indicate the exact affinities. Ver. (Ariel) Conyheari, 

 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 pike are 

 developed early, possibly some form of Comjbeari. 



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 atteauated or cylindrical whorls. 



None of the shells of the Levis Stock are directly or indirectly traceable 

 to any known form of Psil. planorbe, var. plicaium. Their gradations and radical 

 forms, nevertheless, do indicate derivation from planorbe, var. leve, and we must 

 therefore consider the keels, channels, pilge, 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 pilae. The true pilse appear after the 

 keel, and then in some specimens well defined channels arise. 



21 



