HARDWICKE'S SCIENCE-GOSSIP. 



57 



•concealed by the succeeding volution. The aperture 

 is very oblique and bluntly heart-shaped. In the 

 general form and rounded whorls, this species re- 

 sembles P. spamacensis, 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. 

 Planorbis obtusus is found in the upper eocene forma- 

 tion, at Sconce and Headon Hill, Isle of Wight, and 

 at Hordwell, Hants. 



Planorbis euomphalus, Sowerby. (Fig. 26, 2, a. b.) 

 — This well-known species, which appears to be con- 



but as they approach maturity it becomes more and 

 more obtuse, and frequently disappears ; the whorls 

 then assume a transversely oval form, approaching 

 nearly to that presented by P. rotunda/us. On the 

 under surface, the inner margin of the whorls is 

 bent rather suddenly towards the preceding whorl, 

 and presents, in consequence, an obtuse angle, which 

 runs round and defines the umbilicus. Transverse 

 lines of growth are very conspicuous ; and frequently 

 the surface of the shell also presents more or less 

 numerous concentric raised lines, some of which are 

 larger and more prominent than the rest. The aper- 



Fig. 26.-Fossil fresh-water shells. 1. (a and b) Planorbis obtusus; 2. (2) P. platystoma; 3. («) side view of adult shell ; (b) view 



of upper disc of P. euomphalus; 4. P. discus; 5. P. lens; 6. P. elegaus. 



fined to the] fresh-water formations, is equally dis- 

 tinguished from the other Planorbes. It is a large 

 discoidal shell, nearly Hat on the upper face, and 

 presenting a wide and "deep umbilical cavity beneath ; 

 the six or seven volutions of which the shell is formed 

 are fiat or nearly so above, concave below, and each 

 but slightly embracing the preceding volution ; the 

 apex is concealed by the volution of the whorls. A 

 clearly defined and rather acute angle, almost 

 forming a keel, runs round the periphery of.the shell, 

 and separates the upper from the under disc. This 

 angle is always very conspicuous in the young shells, 



ture is subtrigonal, slightly impinged upon by the 

 preceding volution, and very oblique. This species 

 is found in the fresh-water limestone of the Upper 

 Eocene formation, at Headon Hill, Isle of Wight. 



Planorbis rotundatus, Braid.— A rather large shell, 

 formed of six or seven nearly round volutions, 

 separated by a deep suture, and each slightly im- 

 pinged upon by the preceding one ; the volutions are 

 flatly convex on the upper side, convex beneath, and 

 (like those of P. euomphalus) present an angle 

 running round the inner margin near the umbilicus, 

 which as the shell matures becomes almost obsolete. 



