82 EOCENE MOLLUSCA. 
Lymyma, J. Sowerby, 1818; De Blainville, 1825; Desmarest. 
Limnga, G. Sowerby, 1822; Fleming, 1828; G. Sowerby, Jun., 1840. 
— Swainson, 1837. 
LEPTOLIMNEA, Swainson, 1840. 
Lymnopuysa, Fitzinger, 1833. 
LymnuLa, Rafinesque, 1819. 
Gen. Char.—Shell ovate or elongated, frequently turreted, generally thin, smooth ; 
spire always apparent, more or less elevated: volutions convex, somewhat depressed, 
sometimes ventricose, and rapidly enlarging; aperture large, entire, longitudinal, 
ovate, with a tortuous columella bearing an oblique fold; peristome sharp edged. 
The shells forming this genus, constituted part of the genus Bu/imus of Scopoli and 
of Bruguiére ; they had previously been separated by Miiller from the other land and 
freshwater Molluscs under the generic name Buccinum, applied to them by Lister and 
Geoffroy. In lieu of this name, which has been applied by Linnzus to a group of marine 
branchiate Molluscs, Lamarck substituted that of Zymuea, etymologically Limnzea. 
The animal carries on its head two compressed triangular tentacles, enlarged at 
their bases, at the inner and anterior parts of which the eyes are placed. Like most 
others of this order, the Zimnee are hermaphrodite, and although the union of two 
individuals is necessary for fecundation, as among the Helicide, yet impregnation is 
not mutual, as in that group; but the same animal performs the male and female 
functions successively with different individuals. 
The genus, as at present defined, is composed exclusively of the thick dextral 
shells, with a fold on the columella, in which the inner lip is not extended over the 
body whorl; the genus Amphipeplea, (Nillson, the MS. genus Myzras of Dr. Leach,) 
having been proposed for the dextral forms with a plaited columella, in which the 
shell is thin and polished, and the inner lip expanded. The sinistral forms, without 
the columellar fold, have been separated under the generic names Physa (Draparnaud), 
and Aplerus (Fleming), the Bu/inus of Adanson. The propriety of these subdivisions is 
questioned by Mr. G. Sowerby in his ‘ Genera of Shells ;’ but, besides the conchological 
differences above mentioned, there are zoological distinctions which are generally 
admitted as sufficient grounds for retaining them. These are, in Physa and Amphipeplea, 
the condition of the mantle, the edge of which is lobed and capable of extension, so as 
to cover the shell, which thence acquires the polished and shining surface characteristic 
of those genera; and the form of the tentacles, which are elongated and filiform, and 
not thick and triangular, as in the present genus. In Aplerus the edge of the mantle 
is, as in Limnea, simple and not extendible over the shell; that genus, therefore, bears 
the same relation to Physa which Limnea bears to Amphipeplea.* 
* The propriety of these divisions is, to some extent, confirmed by the observations of Mr. W. Thompson, 
to which I have before referred. That author, speaking of the dentition in the different genera of the 
Pulmonata, states that “the character of Limneus appears to be to have one small central tubercle, as it were, 
