48 MALACOZOA. GASTEROPODA. PULMOBRANCHIATA. AQUATICA, 



thickened, incomplete. Vertigo, a whirling or turning 

 round. Muller. 



1. Vertigo edentula. — Shell ovato-cylindrical, sub- 

 conical, with the second turn much larger than the 

 first, greatly exceeded by the third, which is little less 

 than the fourth, the fifth not much larger. JEdentulus, 

 toothless. 



Genus 9. Clausilia. — Shell slender, turrite, subfusi- 

 form, thin, of numerous generally reversed turns, the 

 last smaller than the next, the aperture suboval, ob- 

 lique, with the peristome continuous, free, marginate, 

 and toothed ; the throat with an internal plait capable 

 of closing the cavity. Claudo, to close. Draparnaud. 



1. ClausWia perversa. — Shell turrito-fusiform, rather 

 glossy, with from ten to twelve little convex, trans- 

 versely sulcato-striate turns, the last narrowed, with 

 two wide grooves, the aperture subovate, narrowed near 

 the upper end, and having generally two plaits on the 

 columella. Perversus, turned the wrong way. 



Genus 10. Balea. — Shell slender, turrite, thin, of nu- 

 merous reversed turns, the last proportionally larger than 

 the next, the aperture roundish or oval with the peris- 

 tome thin and simple. Name unintelligible. Leach, 

 Gray. 



1. Balea perversa. — Shell oblongo-turrite, reversed, 

 slender, with the aperture subovate, the peristome thin, 

 simple, the umbilicus distinct. Perversus, turned the 

 wrong way. 



Aquatica. Residing in fresh water. 

 Family III. — Limn^eina. 

 Body ovate or elongated, spiral or conical, with a 

 spiral or conical shell ; the head surmounted by a large 

 expansion or veil ; two contractile tentacula, having the 

 eyes situated at their base. Limnacea, Blainville. 

 Lhnnceadce, Gray. Named from Limnceus, one of the 

 genera. 



Genus 1. Limn^us. — Head with two flattened, sub- 



