[30 J 



HELICELLID^. 



Shell discoidal, orbicular or troohiform, corneous, thin, pol- 

 ished, sometimes transversely striate, translucent or transpa- 

 rent, lip sharp (not reflected outwards, nor internally thick- 

 ened); aperture without proper marginal teeth, but sometimes 

 with internal laminae not reaching to the edge. 



Animal long and narrow. Buccal plate thin, , crescentic, 

 with an elevation in the middle of the cutting edge, side 

 slightly striate in the centre, or all over. 



Lingual dentition. — Uncini long and broad, tridentate, late- 

 rals hug, narrow, curved, bidentate. 



Suh-families. 



VlTEiNiNiE. Shell depressed, very fragile, consisting of 

 about three whorls, the last extremely enlarged ; mouth very 

 oblique and large, extending to the centre of the base of the 

 shell. 



Animal too large for complete retraction within the shell. 



Some of the species of Yitrina, as well as Helix, have a 

 caudal mucous gland, and would, therefore, in accordance 

 with the views of Gray and others, be placed in another fam- 

 ily. We are at present compelled to consider the gland as of 

 no importance whatever in classification, or else to construct an 

 exceedingly artificial and unnatural system. 



Helicellixje. Shell thin, glabrous, translucent or transpa- 

 rent, polished, glohosely depressed; mouth not dentate. Um- 

 bilicus generally narrowly perforate. Umbilical region im- 

 ressed. 



Lingual dentition. — ^As in Vitrininse. 



Differs from Vitrininse in the moderate aperture and im- 

 pressed umbilical region, from Gastrodontinse in being more 

 depressed,, and not impressed striate, and from Patulinee in the 

 absence of opaque color, or ribs. 



Gastrodontinse.* Shell thin, translucent, striate or ribbed, 

 generally depressed conical, frequently lamellately toothed. 



Lingual dentition. — Generally as in the above, sometimes 

 the laterals are square, bidentate. 



Distinguished from all the others by conical shape, from 

 Patulinee, also, by its narrow umbilicus, and diaphanous tex- 

 ture. 



* This and the following Sub-family are not proposed wilh any intention 

 but to facilitate the determination of species. The Sub-family Valloiiiiios 

 of Mr. Morse, in its premt limits, we cannot adopt. 



