106 EOCENE MOLLUSCA. 
No. 58. PranorBis TRoPIS. Ff. #. Hdwards. Tab. XV, fig. 10 a—d. 
P. testé minutd, depressd, levi, superne parum concavd, subtus late umbilicata : anfrac- 
tibus quaternis vel quinis, lente crescentibus, supra convexiusculis, infra sub-planis, ad 
peripheriam carinatis, singulo dimidium antecedentis obtegenti: apertura obliqua, angusti- 
cordatia. 
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 
and carinated as the English specimens, which more resemble. the P. evacutus (Gould).” 
I have not seen any French specimens of P. planulatus; 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. ¢ropis 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 /ee/ 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. evacutus; 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. Sowerby. Tab. XV, fig. 11 a—d. 
PranorBis Hemistoma. Sowerby. 1818. Min. Conch., vol. ii, p. 91; t. 140, fig. 6. 
P. testa minutd, depressd, levi, superne profundé cavatd, subtus subpland, ter quaterve 
circumvolutd ; anfractibus vie involventibus, ad peripheriam sub-angulatis, supra conveat- 
