102 EOCENE MOLLUSCA. 



No. 53. Planorbis obtusus. Sowerby. Tab. XV, fig. 1 a — e. 



Planokbis obtusus. Sow. 1818. Min. Con., vol. ii, p. 91, t. 110, fig. 3. 



P. testa depressd, Icevi ; superne convexiusculd, subtus concavd : anfractibus quints, 

 obtuse-ovalibus, sesepartim involvenlibus ; suturisprofundis: aperturd per-obliqud, obcordatd. 



This very pretty shell has a smooth, shining, nearly polished surface ; it is depressed, 

 nearly flat on the upper side, and moderately concave beneath. The volutions are five, 

 separated by a deep suture, slightly convex on both faces, but rather more compressed 

 on the under side than on the upper, obtusely rounded at the periphery, and each 

 nearly half concealed by the succeeding volution. The aperture is very oblique and 

 bluntly heart-shaped. 



In the general form and rounded whorls, this species much resembles P. sparna- 

 censis (Desh.), but the whorls are not so numerous, and the aperture is more oblique. 

 The lenticular form and angulated margin of P. lens, will prevent its being confounded 

 with that species. 



Size. — Diameter, 7-10ths of an inch. 



Localities. — Sconce ; Headon Hill ; Upper mar., Hordwell. 



No. 54. Planorbis discus. F. JE. Edwards. Tab. XV, fig. 7 a — d. 



P. testa valde compressd, quinquies circumvolutd, superne plana, subtus profunde 

 cavatd : anfractibus sex, rapide crescentibus, parum invohentibus, subtus convexiusculis, ad 

 marginem umbilicalem angulatis : aperturd per-obliqud, elongato-cordatd. 



This well-marked species appears to be peculiar to the fresh-water formation at 

 Sconce. It is a much compressed discoidal shell, nearly flat on the upper side and 

 deeply hollowed out below. The five or six whorls of which it is formed enlarge 

 rapidly, and each is slightly embraced by the succeeding one ; they are much flattened 

 above, slightly convex below, and obtusely rounded at the outer edge. In the young 

 state, the inner margin is bent rather abruptly towards the preceding whorl, and pre- 

 sents an obtuse angle which defines the umbilicus, similar to that presented by P. 

 euomphalus and P. rotundatus. As the shell approaches maturity, the under side 

 assumes an almost regularly convex form, and the angle becomes obsolete. The 

 aperture is very oblique, and of an elongated heart-shape. 



The species which most resemble the present are P. euomphalus and P. oligyratus. 

 From the first of these, it is distinguished by its more compressed discoidal form, and 

 the rounded periphery. The whorls also enlarge more rapidly, and are consequently 

 fewer and broader ; and they embrace more of the preceding one than do those of 

 P. euomphalus. The umbilical cavity also is not so wide. It is more difficult to 



