104 EOCENE MOLLUSCA. 



A small smooth shell, formed of three tumid rapidly-enlarging whorls, each 

 embracing about one half of the preceding one ; the whorls are rounded, almost equally 

 convex above and below, but a little compressed on the upper side. The apex is 

 wholly concealed by the involution of the whorls. The aperture is wide, very oblique, 

 and bluntly heart-shaped ; sometimes, in the adult shell, the peristome is reflected. 

 The upper side is slightly concave, and the under side presents a wide and moderately 

 deep umbilicus. 



This species, which appears to have been first noticed by Mr. Wood, is so well 

 marked that it cannot be confounded with any other. It somewhat resembles the 

 young shell of P. oligyratus ; but, in the latter shell, the whorls are not so embracing, 

 the upper side is more flattened, and the under side is not so regularly convex. The 

 aperture, also, is narrower and much less oblique. The reflected lip, which frequently 

 occurs in the present species, evidences the maturity of the shell ; arfd the difference 

 between the size of such specimens and that of the fully formed shell of P. oligyratus 

 confirms the separation of the two species. In addition to these distinctions, it may 

 be stated that the present species is found plentifully both in Headon Hill and at 

 Hordwell ; but that as yet no specimen of P. oligyratus has been found at either of 

 those localities. 



Size. — Diameter 3- 1 Oths of an inch. 



Localities. — Hordwell ; Headon Hill ; and Sconce. 



No. 57. Planorbis lens. Brogniart. Tab. XV, fig. 8 a — d. 



Planorbis lens. Brogn. 1810. Ann. du Mus., vol. xv, p. 372, t. 22, fig. 9. 



— — lb. 1811. Journ. de Phys., &c. vol. lxxii, p. 421. 



— — Fer. 1814. Mem. geol., p. 61, No. 10. 



— — Sow. 1818. Min. Con., vol. ii, p. 91, t. 140, fig. 4. 



— — Desk. 1824 — 37. Desc. des coq. foss., &c, vol. ii, p. 87, t. 9, 



figs. 11—13. 



— — Bouill. 1836. Catal. des coq. foss. de l'Auvergne, p. 115. 



— — Desk. Ency. Meth. Vers., &c., vol. iii, p. 783, No. 16. 



— — lb. 1838. Lam. Hist. Nat., 2d edit., vol. viii, p. 396, No. 7. 

 — Morris. 1843. Cat. Brit. Foss., p. 156. 



An. planobbe angueeux? Brard. 1810. Ann. du Mus., vol. xiv, p. 435, t. 27, figs. 23-4. 



P. testa parvd, lenticulari, lavi, utrinque parum, sed plus superne quam subtus, cavatd; 

 anfractibus quaternis, singulo fere dimidium antecedentis involventi ; supra convexiusculis, 

 subtus plano-convexis, ad peripheriam acute angulatis ; aperturd parum obliqud, elongato- 

 cordatd, 



Mr. Sowerby, when he gave the name P. lens to a British fossil, appears not to 

 have been aware that that name had previously been dedicated by Brogniart to a 

 shell from the Paris basin ; but, by a singular coincidence, the shells described by 



