FOURTH, OB < "I:- BRAXCH 161 



ies not Appear to indicate the exact affinities I I I ybeari, 



1. all seem 

 the youn oi Verm is, in w i h keel and j>ih are 



ilj some !'<>: : 



LEVIS STOCK. 



The living chambers may be cylindrical, or they ma\ be broadened oul and 

 ;. modified by the growth of the whorl, but the length is invariably 



under one volution, and often does not exc 1 one half of a volution. There 



in be 1 1 « • neci elation between ili«' Bhape of 1 1 1 « - chamber and its 



: tin- inii-t attenuated and cylindrical whorla have nol invariably the 



-• living chambers, nor the broadest whorls invariably the shortest living 



chambers. It is however true, in a very general way, that the longest living 



chain' r in the Plicatus Stock among the Bpeciea which have 



the iii- » - t attenuated <>r cylindrical whorls. 



N • :" the sheila of the Levis St «k are directly or indirectly traceable 



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



forms, in-. in indicate derivation from pbutorbe, var. leve, and we must 



therefore consider the keels, channels, pilae, and BUtures which arc similar in 



the t i- having originated independently in each Btock, or, in other 



. as homoplastic, and not homogenous charai 



higher - - tend to become involute, and to eloi 



bdomino-dorsal diameter of the whorl. 

 Tie are almost purely arietian in proportions and outline-, the 



auxiliaries are rarely or never inclined posteriorly, the marginal digitations are 



lies broader and less dendritic than in the PI 



FOURTH, OB CORONICER W BR LNCH. 



I, and involution i» limited t>> the area of the abdo- 



l 1 •inn the > 1 1 • ■ 1 1 is apt to acquire flattened smooth 



mil a narrow abdomen, but n< the keel, nor becomes rounded on 



• nit of involut ion Flattened sidi 

 i abdomens m ippear in the adult 



ARNIOCERAS. 



The in- : ma) !>•■ recognised l»\ the smoothness and thin, 



; i in form "i the lii-t three or lour whorla The keel 



. which develops lal true keel Tl folds 



in th<- young, which into piln I fter the 



keel, and then in- will defined channels ai 



