106 EOCENE MOLLUSC A. 



No. 58. Planorbis tropis. F. E. Edwards. Tab. XV, fig. 10 a — d. 



P. testa minutd, depressd, lavi, superne parum concavd, subtus late umbilicatd : anfrac- 

 tibus guaternis vel quints, lente crescentibus, supra convexiusculis, infra sub-planis, ad 

 peripheriam carinatis, singulo dimidium antecedentis obtegenti : aperturd obliqud, angusti- 

 cordatd. 



Mr. Wood ('London Geol. Journ.,' vol. i, p. 118,) has referred this species to 

 P. planulatus (Desh.), observing, however, that " the figure by Deshayes is not so flat 

 andcarinated as the English specimens, which more resemble the P. exacutus (Gould)." 

 I have not seen any French specimens of P. planidatus ; but, on comparing the 

 English shells with the description and figures given by M. Deshayes, I cannot concur 

 in referring them to that species. P. tropis is a minute, compressed, polished 

 shell, a little sunk round the apex, and widely but not deeply umbilicated ; the four 

 or five volutions, of which it is formed, are slightly convex above, nearly flat beneath, 

 and compressed near the outer margin so as to present a prominent keel, which runs 

 round the periphery a little below the middle of the whorl. The whorls are much 

 concealed, each embracing about one half of the preceding one ; the aperture is very 

 oblique and of a longish heart shape. 



Although the general resemblance between this species and P. planulatus must be 

 admitted, yet there are, I think, sufficient grounds for specific distinction. In the 

 present species, the whorls increase more slowly and are more concealed ; they are not 

 so convex above nor so flat beneath, and, consequently, the keel is near the middle of 

 the shell, and the aperture assumes an elongated heart shape ; whereas, in P. planu- 

 latus, owing to the greater flatness of the under surface of the whorls, the marginal 

 angle (for, judging from the figure given by M. Deshayes, the term keel is not 

 applicable,) runs round the base of the shell, and the aperture is sub-trigonal. The 

 English shell also appears to be smaller than the French one. On these grounds, 

 notwithstanding the distrust I feel at dissenting from Mr. Wood's opinion, I consider 

 the present to be a distinct species. In the character and position of the keel, P. tropis 

 corresponds with P. exacutus; but in the recent shell, the whorls enlarge more rapidly 

 and are more convex, both above and below; the umbilicus is deeper, and the aperture 

 wider, than in the present species. 



Size. — Diameter, not quite 3-20ths of an inch. 



Locality. — Hordwell. 



No. 50. Planorbis hemistoma. Sowerbg. Tab. XV, fig. 11 a — d. 



Planorbis Hemistoma. Sowerby. 1818. Min. Conch., vol. ii, p. 91 ; t. 140, fig. 6. 



P. testa minutd, depressd, lavi, superne profunde cavatd, subtus subpland, ter quaterve 

 circumvolutd ; anfractibus vix involventibus, ad peripheriam sub-angulatis, supra convexi- 



