82 EOCENE MOLLUSCA. 



LymNjEA, J. Sowerby, 1818; De Blainville, 1825; Desmarest. 

 Limnea, G. Sowerby, 1822; Fleming, 1828; G. Sowerby, Jun., 1840. 



— Swainson, 1837. 

 Leptolimnea, Swainson, 1840. 

 Lymnophysa, 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 Bulimus of Scopoli and 

 of Bruguiere ; they had previously been separated by Muller 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 Linnaeus to a group of marine 

 branchiate Molluscs, Lamarck substituted that of Lymnea, etymologically Limnaea. 



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 Limnaa are hermaphrodite, and although the union of two 

 individuals is necessary for fecundation, as among the Helicidce, 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 Myxas 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 Aplexus (Fleming), the Bulinus 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 Aplexus the edge of the mantle 

 is, as in Limnaa, simple and not extendible over the shell; that genus, therefore, bears 

 the same relation to Physa which Limncea 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 Limnceus appears to be to have one small central tubercle, as it were, 



