1849.] MORRIS ON NERITOMA. 333 



Genera in which the outer lip has 



j^ 



A greater or less sinus. No sinus. 



Neritoma. Nerita. 



Amphibola. Ampullaria. 



Clithon. Neritina. 



Platychisma. Trochus. 



Pleurotomaria. Trochus. 



Acroculia. Pileopsis. 



Pleurotoma. Fusus. 



Murchisonia. Cerithium. 



Emarginula. Patella. 



In consulting the above table of the two groups, it will be remarked 

 that most of those fornis furnished with a sinus belong to extinct 

 genera : thus Acroculia, Murchisonia, Platychisma, are found in the 

 palaeozoic series, Pleurotomaria and Neritoma in the secondary strata, 

 Pleurotoma in the tertiary and also recent. 



Trachelipoda, Lam. 



Family Neritaces, Lam. 



Neritoma. 



Testa ventricosa, crassiuscula, Iseviuscula, epidermide induta, non 

 umbilicata, spira brevi obtusa ; anfractibus subcarinatis, ultimo 

 ventricoso ; apertura subovali, obliqua ; labro acuto, bisinuato ; 

 labio incrassato, planulato, superne canalifero, non denticulato nee 

 crenulato ; impressione musculari elongato-ovata. 



A ventricose and moderately thick shell, nearly smooth, or merely 

 marked by the lines of growth, having a slightly elevated spire and 

 three or four subangular volutions ; not umbilicated ; aperture ovately 

 oblong, outer lip thin, sharp and bisinuate, with one angular sinus 

 towards the middle of the shell corresponding to the carina and a 

 rounded sinus near the base ; inner lip broad, flat, and thickened, 

 slightly depressed in the middle, and not crenulated nor denticulated. 



The above characters sufficiently distinguish this genus ixoxaNerita, 

 in which also the inner lip is much less confluent with the outer one 

 than in Neritoma. The flattened and expanded columellar lip, and 

 not being umbilicate, remove it from Amphibola. 



From Neritopsis, which connects Nerita with Natica, it is separated 

 by the above-mentioned characters, as well as in not having a deeply 

 notched inner lip, as in that genus. If with Dr. Grateloup and 

 M. Pictet we exclude Natica as belonging to a distinct family, the 

 Neritaces will be neatly limited to the following genera : Navicella, 

 Pileolus, Nerita, Neritina, Neritopsis, and Neritoma. 



The species belonging to Neritoma have at present only been 

 found in the oolite. The shell from which the above generic descrip- 

 tion was formed, was obtained by Mr. Lowe from the upper beds of 

 the Portland series, at Swindon, Wilts. This specimen is interesting, 

 as exhibiting distinctly the coloured marking upon the surface. 



2 A 2 



