98 GASTEROPODA. LIMNAD&. 
glossy, and finely striated transversely. Aperture with four 
folds behind, and several tubercles in front. Height half an 
inch. 
This shell varies very much in size ; and in the young state 
has fewer teeth, or folds. In adult shells, the posterior aspect 
of the aperture has four folds; the first, on the left, coalescing 
with the peritreme ; the second, largest ; the third and fourth, 
gradually smaller. The tubercles, on the anterior aspect, are 
very irregular in size, with regard to each other and in number. 
I have counted eight in one adult, and only three in another. 
These tubercles, as well as all the folds, are white. 
This species seems to be entirely confined to estuaries ; it'is 
very common in Mill Bay ; and at the mouth of the Tamar, 
near Plymouth ; at the mouth of the river, near Barnstaple, in 
Devon; at the mouth of the Southampton river ; and at Ports- 
mouth. I found it likewise at the mouth of the Cork river; 
and at Dungarvon, in Ireland, in abundance. My friend, Mr. 
Stephens, informs me, that he has found dead specimens 
amongst the rejectamenta of the Thames as high as Battersea. 
Hence it may be inferred to inhabit the mouth of that river. 
FAMILY IV. LIMNADA. 
Pulmonés aquatiques, Cuvier. 
Tentacula duo interné ad basin oculigera; ad apicem seepits 
acuminata. Testa turrito- aut involuto-spiralis. Apertura 
simplex, mermis. 
Habitant in stagnis, paludibus, et in aquis lenté fluentibus ; 
phytophagee ; aéra elasticum respirantes. 
Stirpium et Generum Synopsis. 
Stirps 1. Testa dextrorsum elevato-spiralis. Ultimus anfracti- 
bus maximus. Tentacula deplanata, ad apicem gradatim 
acuminata. 
