PULMONATA. 103 



separate the present species, in the young state, from P. oligijratus ; 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. E. Edwards. Tab. XV, fig. 3 a — e. 



P. testa st^rne sub-pland, subtus cavatd: anfractibus quaternis, rapide crescentibus, 

 parum involventibus, ad perip/ieriam obtuse angulatis : subtus ad marginem umbilicalem 

 angulatis : aperturd sub-trigond, parum obliqud. 



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 platystoma. S. Wood. Tab. XV, fig. 2« — d. 



Planorbis platystoma. S. Wood. Lond. Geol. Joum., vol. i, p. 118. 



P. testa parvd, utrinque fere equaliter cavatd; anfractibus trinis, rotundatis, 

 tumidis, rapide crescentibus, supra depressiusculis, subtus convexis, singulo fere dimidium 

 antecedentis involventi : aperturd magna, per-obliqud, obtuse obcordatd, peristomate 

 aliquando reflexo. 



