338 EOCENE MOLLUSCA. 
No. 261. Bytuinta conica ? Prevost. Tab. XXXIV, fig. 8 a, 6. 
PaLupina conica, Prév. Journ. de Phys., p. 11, 1821. 
_— _— Desh. Cog. foss. des Env. de Par., t. xi, p. 129, pl. xvi, fig. 7, 1824. 
— ?impvrata, S. Wood. Lond. Geol. Journ., p. 118, 1846. 
Birninta conrca, Desh. An.sans Vert. du Bas. de Par., t. ii, p. 494, 1858. 
Hyprosra (Brruynta) conica, J. Lowry. Chart Brit. Tert. Foss. pl. ii, 1866. 
AssIMINEA contca, Sandb. Land- und Siissw.-Conch., p. 212, t. xi, fig. 11, 1872. 
Spec. Char. “ B. Testa ovato-conicd, levigutissimad, acuminata, anfractibus planu- 
latis, suturd superficial separatis ; apertura ovato-angulatd ; marginibus acutis.’—Desh. 
Height, ~5ths of an inch. 
Locality, Britain: Hordle (S. Wood), Headon Hill (Ldwards). 
France: Vaugirard (Deshayes). 
I have copied the specific diagnosis as given by Deshayes for B. conica which seems to 
correspond so precisely with that of our fossil that I think the two may fairly be referred 
to one and the same species. Many years ago I found a specimen of this genus at Hordle 
to which I gave the name of Paludina? impurata from its very close resemblance to the 
one so common in our own freshwaters, and so very abundant in the freshwater deposits 
of our Upper Tertiaries, at Grays and Clacton (Paludina impura, Brard, Helix tentaculata, 
Linné). 
The figures given in my plate are somewhat enlarged. This species may probably 
be variable where a large number of individuals are found like these specimens of B. 
tentaculata so numerous at Grays and Clacton, where they show a considerable variation 
in their proportionate dimensions, some being much more elongated than others, a varia- 
tion which produces one also in the tumidity of the volutions. 
Dr. Sandberger has referred this to Assiminea, a genus proposed by Dr. Leach 
for a shell found in the Woolwich marshes and sent by him to the late Dr. Fleming 
with the name Assiminea Grayana “as the type of a new freshwater genus,’ ‘ Hist. Brit. 
An., p. 275. This shell much resembles that of Bythinia, but it is described as having 
an operculum that is spiral, while that of By/Ainia is increased by concentric layers; and 
although we have not the operculum of this shell to guide us, it seems so very closely to 
resemble our cominon shell Bythinia tentaculata, that I think it must belong to the same 
genus. 
‘The operculum when it can be obtained is, I think, a good character to assist in 
establishing a genus, but the operculum of our species has not yet been found. In 
Paludina and Bythinia the operculum is increased by concentric layers, while in /Hydrodia, 
Nematura, and Assiminea it is spiral. A small shell, Vematura pupa, Nyst, is very abundant 
in our Upper Eocene Beds at Headon Hill, but its operculum has not yet been found, as 
in all probability it was a corneous one. The late Mr. G. B. Sowerby figured and 
