PULMONATA. 103 
separate the present species, in the young state, from P. oligyratus ; but, in the latter, 
the axis is longer, the whorls are more convex on the upper side, and the obtuse angle 
which runs round the periphery, near the upper surface, gives a subtrigonal form to 
the whorls and the aperture, quite distinct from the transversely oval form in the 
present species. 
Casts of this Planorbis occur in great abundance, but specimens with the shell 
preserved are extremely rare. 
Size.—Diameter, 1 inch and 6-10ths. 
Locality Sconce. 
No 55. PLANoRBIS OLIGYRATUS. F. HL. Edwards. Tab. XV, fig. 3 a—e. 
P. testdé superné sub-pland, subtus cavatd: anfractibus quaternis, rapide crescentibus, 
parum involventibus, ad peripheriam obtuse angulatis: subtus ad marginem umbilicalem 
angulatis : apertura sub-trigond, parum obliquad. 
The present, like the preceding species, is apparently confined to the fresh-water 
formation at Sconce; but it is not so plentiful as P. discus. It is a moderately-sized 
shell, formed of four rapidly increasing whorls, somewhat convex above, and rather 
acutely angulated round the inner margins on the under side, just above the umbilicus. 
The periphery, near the upper surface, presents an obtuse angle, from which the 
whorls slope rather abruptly towards the umbilicus, assuming, in consequence, a sub- 
trigonal form. The aperture is slightly oblique and bluntly obcordate, in consequence 
of its being impinged upon by the preceding whorl. The umbilical cavity is deep, but 
not very wide. 
This species somewhat resembles the young shell of P. discus; but the greater 
convexity of the under side of the whorls, and the small degree of obliquity, and the 
shape of the aperture, will distinguish it. The adult shell of P. discus is easily sepa- 
rated by its greater size and compression. 
The specimens ordinarily found are, like those of the preceding species, casts 
merely; with the shell preserved they are very rare. ~ 
Size.-—Diameter, 7-10ths of an inch. 
Locality.— Sconce. 
No. 56. PLANORBIS PLATYSTOoMA. WS. Wood. Tab. XV, fig. 2 a—d. 
PLaNnorsis PLatystoma. S. Wood. Lond. Geol. Journ., vol. i, p. 118. 
P. testé parva, utrinque fere equaliter cavatd; anfractibus trinis, rotundatis, 
tumidis, rapide crescentibus, supra depressiusculis, subtus conveais, singulo fere dimidium 
antecedentis involventi: apertura magnd, per-obliqui, obtuse obcordatd, peristomate 
aliquando reflexo. 
