116 



XXI. SEGMENTINA. 



Shell orbicular, somewhat discoid : aperture oval, 

 semiconcamerated within ; the partitions transverse, 

 and resembhng a triradiate opening. 



99. Segmentina nitida. Flem. Ed'inh. Encyclop. vii. 



Planorbis nitidus. Drap. p. 46. t. 2./. 17 — 19. 

 Hemithalamus. Leach, Mollusc, p. 137. 

 Nautilus lacustris. Mo?it. p. 191. t. 6./. 3. 



Shell hardly a quarter of an inch in diameter, highly 

 polished and smooth, of a chestnut or reddish- 

 brown colour, flattish and semitransparent : volu- 

 tions four, the outer one very large in proportion, 

 and marked with two or three whitish transverse 

 lines exhibiting the internal partitions ; the upper 

 surface very convex, with a deep umbilicus in the 

 centre, the under side nearly flat, and umbilicate 

 in the centre ; the circumference slightly carinate : 

 aperture oval, inclining to triangular, with the peri- 

 treme interrupted. 



Except for its internal semiconcamerated partition, 

 this shell exactly resembles the Planorbis fontanus. 



In stagnant waters, on aquatic plants. 



