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EOCENE MOLLUSCA. 



No. 261. Bythinia conica ? Prevost. Tab. XXXIV, fig. 8 a, b. 



Paludina conica, PrSv. Journ. de Phys., p. 11, 1821. 



— — Desk. Coq. foss. des Env. de Par., t. xi, p. 1 29, pi. xvi, fig. 7, 1 824. 



— ? impurata, S. Wood. Loud. Geol. Journ., p. 118, 1846. 

 Bithinia conica, Desk. An. sans Vert, du Bas. de Par., t. ii, p. 494, 1858. 

 Hydrobia (Bithynia) conica, J. Lowry. Chart Brit. Tert. Foss. pi. ii, 1866. 

 Assiminea conica, Sandb. Land- und Siissw.-Conch., p. 212, t. xi, fig. 11, 1872. 



Spec. Char. " B. Testa ovato-conicd, laviyatissimd, acuminata, anfractibus planu- 

 latis, suturd superficiali separatis ; aperturd ovato-angulatd ; marginibus acutis." — Desk. 

 Height, i^ths of an inch. 



Locality, Britain : Hordle (S. Wood), Headon Hill {Edwards). 

 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, 

 Linne). 



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 Bythinia 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 common 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 Hydrobia, 

 Nematura, and Assiminea it is spiral. A small shell, Nematura 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 



