THE GASTROPODA 107 



Fust{s is a marine genus, now confined to sub-tropical 

 seas, but, as in so many other cases, found in Britain in 

 the Eocene period. It is rare in earlier formations, but 

 is recorded as far down as the Upper Jurassic. 



8. Planorbis discus (Fig. 33) is a common fossil in 

 the Bembridge Limestone (Lower Oligocene or Sannoi- 

 sian) of the Isle of Wight. The upper surface is flat 

 (spiral angle 180), with a rather deep hollow in the centre. 



FlG. 32. FUSUS PORRECTUS, SOLANDER, EOCENE. 



Left-hand figure, natural size (ornament omitted from earlier whorls) ; 

 right-hand figure, protoconch and post-embiyonic \vhoil. ( x 18.) 

 (Original.) 



The greatest diameter (30 mm. in full-grown shells) can 

 almost be measured on this upper surface, the margin 

 being rounded and the outline below quickly turning 

 inwards at an angle of 35 to the upper surface ; in the 

 centre of the under surface is a shallow umbilicus, about 

 one-third the diameter of the shell. The aperture for 

 two-thirds of its extent has an elliptical outline, but 

 in contact with the inner whorl this changes to a concave 



