156 
rocky flats wliicli are exposed in a low state of the water. Its elon- 
gated form is somewliat similar to tkat of tlie rectus, Lam. ; but it 
is muck tkinner, more widely rounded before^ and more compressed, 
particularly anteriorly. Its teetk distinguisk it from any otker 
species kitkerto described. Old shells are very muck decorticated 
and eroded ; and in some individuals^ tke impressed line on tke lat- 
eral tootk is kardly perceptible : still it cannot for one moment be 
mistaken for an Alasmodonta, as tke kabit is quite different. Mr. 
Lesueur kas presented me witk a specimen found in tke Wabask. 
PI. 6. 
Delphintjla.— -Skell spiral^ subdiscoidal or conic^ umbilicated * 
wkorls rougk^ foliaceous or angulated on tke surface ; aperture orbi- 
cular^ entire ; labia united^ often fringed or furnished witk a peris- 
tome. 
Ohs. Linne referred tke laciniata and distorta, tke only species 
known to kinq to tke genus Turho, in consequence of tke rotundity 
of tke aperture ] but Lamarck separated them, and witk a number 
of new species, chiefly fossil, formed tke present group. Lamarck 
observes that they evidently approach tke Scalarise,” and among 
them we also see species, of which tke spire is loose, and tke volu- 
tions separate. In addition to tke above mentioned species, Lamarck 
gives one or two others and about ten fossil ones. Defrance enu- 
merates about thirty species. 
Delphinula ? LAXA. — Specific character. Whirls all separate 
and distinct. PI. 7. 
Delphinula laxa, nohis. Journ. Acad. Nat. Sci., vol. 5, p. 207. 
Melania.— -Shell turrited, rather thick, covered by an epi- 
dermis; spire generally elongated; aperture ovate or oblong, longer 
than broad, acute above; labrum simple, acute, prominent near 
tke base and rather abruptly retracted at its junction witk tke base 
of tke columella, and not united above to tke labium ; columella 
destitute of a fold, arquated towards tke base ; operculum corne- 
ous ; umbilicus none. 
Ohs. Before Lamarck perceived tke necessity of establishing 
this group, some of tke species were variously arranged. Linne, 
Lister, Sckroeter, and others, considered them as belonging to 
Helix ; Muller to Nerita and Buccinum ; Brugui^re to Bulimus, 
and Chemnitz referred one species to Stromhus. Tke genus differs 
from Helix, Lymneus, &c., by having an operculum as in Paludina, 
