SECOND* MONOGRAPH OF THE GEXUS 



HELICINA, 



INCLUDING THE GENERA TROCHATELLA, LCCIDELLA, HELICINA, 

 SCHIASCHEILA AND ALCADIA, OF AUTHORS. 



Char. Gen. Molluscum terrestre. Testa spiralis, helicoidea, 

 labio interno ab anfractu ultimo baud distincto, plerumque circa 

 columellam calloso. Operculum testaceo-corneum, subquadratum, 

 non spirale. — Land-mollusk. Shell spiral, helicoid, inner lip not 

 distinct from last whorl, generally callous round the columella. 

 Operculum horny-testaceous, rather square, non-spiral. 



My objection to the smaller genera adopted by some authors 

 chiefly arises from the impossibility of defiuiug them. Taking 

 the genus Helicim as above described, its limits are perfectly 

 discernible ; no one having a complete specimen in hand, can 

 doubt whether it be a Helicina or not. The definition includes 

 all that belongs to it, and excludes all that is foreign to it. This 

 caunot be said of auy description that has been or thnt can be 

 given to Trochatella, Alcadla, etc. The notch between the colu- 

 mella and base of the outer lip, upon which the latter genus is 

 founded, is a thing of infinitesimal degrees, from a little depres- 

 sion in front of a thick tooth to a spirally incised slit, and thence 

 to a scarcely perceptible sinus in the thin peritreme of some 

 smaller species. (See Plates I. and II.) The chief character of 

 Trochatella is the absence of callus at the base of the lower 

 whorl, but if that were made the basis of division it would in- 



* The number of species contained in the former monograph is so small com- 

 pared with those which are now known, that it has been thought better to 

 make the present monograph complete, even at the inconvenience of repeating 

 the figures of the former seventy species. The advantage of this will be to 

 preserve to the eve the entire series in the order of their adinities. Subscri- 

 bers will not lose by this, as we have compensated by giving them a consider- 

 able number of extra figures in the present double part . 



VOL. III. 2 N 



