[30 J 



HELICELLID^. 



Shell discoidal, orbicular or trocbiform, corneous, tbin, pol- 

 isbed, sometimes transversely striate, translucent or transpa- 

 rent, lip slimy (not reflected outwards, nor internally tbick- 

 ened); aperture witbout proper marginal teetb, but sometimes 

 witb internal laminae not reacbing to tbe edge. 



Animal long and narrow. Buccal plate tbin, crescentic, 

 witb an elevation in tbe middle of tbe cutting edge, side 

 sligbtly striate in tbe centre, or all over. 



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

 rals long, narrow, curved, bidentate. 



Suh-fci'milies. 



YiTEiNiNJE. Shell depressed, very fragile, consisting of 

 about tbree wborls, tbe last extremely enlarged ; moutb Yerj 

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

 sbell. 



Animal too large for complete retraction witbin tbe sbell. 



Some of tbe species of Yitrina, as well as Selix, bave a 

 caudal mucous gland, and would, tberefore, in accordance 

 witb tbe views of Gray and otbers, be placed in anotber fam- 

 iljf. We are at present compelled to consider tbe gland as of 

 no importance whatever in classification, or else to construct an 

 exceedingly artificial and unnatural system. 



Helicellinje. Shell tbin, glabrous, translucent or transpa- 

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

 bilicus generally narrowly perforate. Umbilical region im- 

 pressed. 



Lingual dentition. — As in Vitrininse. 



Differs from Vitrininse in tbe moderate aperture and im- 

 pressed umbilical region, from Oastrodontinse in being more 

 depressed, and not impressed striate, and ivom Patulinse \n tbe 

 absence of opaque color, or ribs. 



Gastrodontin^.* Shell thin, translucent, striate or ribbed, 

 generally depressed conical^ frequently lamellately tootbed. 



Lingual dentition. — Generally as in tbe above, sometimes 

 tbe laterals are square, bidentate. 



Distinguisbed from all tbe otbers by conical sbape, from 

 Patulin^, also, by its narrow umbilicus, and diapbanous tex-' 

 ture. 



* This and tlie following Sub-family are not proposed with any intention 

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

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



