58 UNIVALVES. 



take away lanthina, unmistakable owing to its violet shell, we are 

 left with six genera : 



Base flat Trochus. 

 Mouth circular Valvata. 

 Pillar folded Limnaa. 

 Pillar grooved Lacuna. 

 Thick and heavy Littorina. 

 Thin and light Helix. 



Those in which the height is greater than the breadth are not so 

 easily disposed of. Some are between double and treble as long as 

 they are broad, some more than treble as long. The longest in our 

 list are Odostomia, Eulima, Aclis, Turritella, and Scalaria. The left- 

 handed upper whorls distinguish Odostomia; Eulima is narrow and 

 pointed as an awl, and highly polished ; Aclis is smooth or faintly 

 ridged and pyramidal in form ; Turritella has bold spiral ridges, and 

 Scalaria prominent longitudinal ribs, added to which Turritella is the 

 longest narrow shell we have. 



The next batch, those having the length between less than three 

 times but more than twice the length*, comprises Odostomia, with its 

 sinistral upper whorls ; Limncea, with its folded pillar ; Bulimus, with 

 its reflected outer lip; and Acicula, with its reflected inner lip, 

 A cicula, better known as A cme, being a land shell with an operculum. 

 We can put the long eight under one heading to simplify matters : 



Upper whorls sinistral Odostomia. 

 Pillar folded Limn&a. 

 Outer lip reflected Bulimus. 

 Inner lip reflected Acicula. 

 Subulate and polished Eulima. 

 Smooth or faintly ridged Aclis. 

 Spirally ridged Turritella. 

 Longitudinally ribbed Scalaria. 



We are left with those longer than they are broad, but not twice 

 so long as they are broad. These we can divide into those having 

 the curve of the mouth broken into by the body whorl, and those in 

 which it is not interfered with ; in other words, those with an 

 incomplete peristome and those with a complete peristome. The 

 incomplete ones are Limncza, Bulimus (as regards obscurus), Neritina, 

 and Jeffreysia, whose differences can be thus summarised : 



Pillar with fold Limnaa. 



Mouth like a horseshoe Bulimus. 



Shell thin, smooth and glossy Jeffreysia. 



Shell semi-globular, mouth semi-circular Neritina. 



Size alone will sort out Jeffreysia, as none of its three species are 

 larger than the tenth of an inch, and the mouth of Neritina and the 

 fold of Limncea leave Bulimus conspicuously alone. 



Those with peristome complete are more numerous. At the first 

 glance we can pick out Lacuna, with its grooved pillar ; Physa, with 



