SEGMENTINA. 245 



f. 6. — Planorbis contortus. AKller, Verm. ii. 162.; Drap. 

 p. 42. t. 1. f. 39—41.; Jeffreys, Linn. Trans, xvi. 383.; 

 Rossm. Icon. Ii. 16. t. 7. f. 117.; TurUm, Man. ed. 1. f. 96. ; 

 Brard, p. 157. t. 6. f. 12—14.; Leach, Sijn. Moll. 112.; 

 Forbes and Hanleij, B. M. iv. 160. t. 126. f. 3. — Helix um- 

 bilicata. Pidteneij, Dorset, t. 20. f. 11. — Helix crassa. Da 

 Costa, B. S. t. 4. f. 11. 



Inliab. ditches. Common. 



Animal grey or blackish. {Sturm, t. 41.) 



Shell about two tenths of an inch in diameter, 

 and one tenth in thickness, brown horn-colour, and, 

 when free from accidental incrustations, exhibiting 

 in water a bronzed or gilt lustre ; volutions five, 

 remarkably compact and equal in size ; the upper 

 surface with a large and deep umbilicus; aperture 

 very narrow, crescent-shaped, wider than high, only 

 slightly produced on the under side. 



In describing this shell, Dr. Turton called that 

 the upper side which was considered the lower in 

 other species, and vice versa. 



9. Segmentina Flem. (Segment Shell.) 



Animal nearly like Planorbis, hut the shell is polished, 

 and the cavity contracted by permanent internal 

 transverse ridges, having a triradiated opening ; 

 lingual teeth very minute. 

 So called from the internal segments or partitions, 

 Mr. Alder does not think that the septa in the 

 shell are sufficient to raise it to the rank of a genus, 

 as the animal is exactly like Planorbis. Miiller con- 

 founded it as a variety with P. fontanus ; like that 

 species, the spire is umbilicated, and the front of the 

 shell has the appearance of being the proper spire. 



B 3 



