112 MOLLUSCA FROM THE CRAG. 



This species was found by Captain Alexander at Bulcham, and by Mr. Wigham 

 near Norwich, but by no means abundantly at either locality. The specimen 

 figured is from the cabinet of Mr. Lyell. There does not appear to be the 

 slightest difference between the few specimens I have seen from the Mammaliferous 

 Crag and those of the present day. This shell is found in great abundance in the 

 truly Fluviatile deposits of Stutton and Clacton, and also its operculum. A slight 

 variation in form exists among these specimens, some being much more ventricose 

 than others, but the same differences may be observed in the recent shells. This 

 species was separated from Paludina by Mr. J. B. Gray, and formed into a genus 

 under the name of BitJdnia, in consequence of its possessing a calcareous operculum, 

 and from the shell being thickened internally behind the peritreme ; these characters, 

 however, can scarcely be considered as more than specific, as the animals are said 

 to be precisely similar, and the different species, like the Naticse, are furnished some 

 with a calcareous, others with only a corneous operculum. 



Valvata, Mutter, 1781. 



Nerita (spec.) Miiller. 

 Turbo (spec.) Mont. 



Gen. Char. Shell conical, or sometimes discoidal, thin, corneous, or semitrans- 

 parent, with rounded or cylindrical volutions, not impressed by the preceding 

 whorl, generally smooth, sometimes carinated; aperture nearly circular, with a 

 sharp and continuous peritreme, not reflected ; operculum corneous. 



This is a true fresh- water genus, and the few individuals found in the Crag 

 associated with marine and estuary shells are, in all probability, transported speci- 

 mens. Some shells of this genus approach so near in form to species of the genus 

 Planorbis as to be almost undistinguishable in a fossil state. It may be remarked 

 that the aperture in this shell is always more distinctly orbicular, and not at all 

 impressed by the previous volution. 



1. Valvata piscinalis. Mutt. Tab. XII, fig. 3. 



Nerita piscinalis. Miiller. Verm. p. 172, No. 358, 1774. 

 Helix piscinalis. Gmel. Syst. p. 3627, 1788. 

 Turbo fontinalis. Mont. Test. Brit. p. 348, 1803. 



— Bon. Brit. Shells, iii, t. 102. 

 Cyclostoma obtusum. Drap. Moll. t. 1, fig. 14, 1805. 

 Valvata piscinalis. Flem. Brit. An. p. 286, 1828. 



— Gray. 2d edit. Turt. Man. pi. 10, fig. 1 14. 



— Dujardin. Mem. Geol. de France, torn, xi, pt. 2. 



— S. Wood. Catalogue 1842. 



— Morris. Catalogue of Brit. Foss. p. 166, 1843. 

 Valvata antiqua. Id. 



— G. Sowerby. Mag. Nat. Hist. 1838, p. 574. 



V. Testa tenui, globoso-conoided, subtrocJdfonni, vel turbinoided, perforata ; spird 

 mediocre elevatd; anfractibus 4 — 5, convexis, rotundatis ; suturd depressd ; aperturd 

 rotunda ; umbilico prof undo. 



