242 LIM1UEAD2E. 



109. 7. PLANORBIS vortex. Whorl Coil Shell. 

 Shell brown, pellucid, thin, flat above, slightly 

 and regularly concave beneath, with six or 

 seven gradually increasing sharply keeled 

 volutions, which are convex before and flat 

 behind ; mouth rhombic, compressed, (t. 8. 

 f. 91.) 



Helix vortex. Linn. S. N. i. 1242.; Mont. T.B. p. 454. t. 25. 



f. 3. Planorbis vortex. Muller, Verm. ii. 158.; var. a., 



Drop. p. 44. t. 2. f. 4, 5. ; Brard, p. 154. t. 6. f. 9. ; Lam. 



Hist. vi. 154.; Jeffreys, Linn. Trans, xvi. 382.; Hossm. Icon. 



104. t. 2. f. 61.; Sturm, Fauna, t. 44.; Turton, Man ed. 1. 



109. f. 91. Leach, Sun. Moll. 112.; Forbes and Hanley, 



B. M. iv. 157. t. 127. f. 6, 7, 8. Helix planorbis. Da 



Costa, p. 65. t. 4. f. 12. Planorbis compressus. Michaud, 



Compl. 81. t. 16. f. 6. 8. Planorbis tenellus. Studer, 



Cat. 92. 

 Var. 1., the mouth of the shell with a thickened internal rib : 



Planorbis leucostomus. Michaud, Compl. 80. t. 16. f. 3, 4, 5.; 



Possm. Icon.'i. 105. f. 62. Planorbis rotundatus. Poir. 



Prod. 93.; Moq. Tand. ii. 435. t. 30. f. 3846. Planorbis 



vortex /3. Drop. Hist. 45. t. 2. f. 6, 7. 



In stagnant waters. 



Animal violet-brown. 



Shell three eighths of an inch in diameter, very 

 flat and thin, with six or seven gradually increasing 

 volutions, slightly concave above, and quite flattened 

 underneath, so as to form a sharp edge round the 

 outer volution ; aperture a* little angular. 



In summer, when the ditches are dry, this animal 

 closes up its shell with a white epiphragm, within 

 which it lies secure under the mud and weeds, in a 

 state of torpidity, until the ditches are again filled 

 with water. The animal then thickens the internal 

 margin of the shell, forming a permanent white rim. 



Miiller long ago pointed out the white and thick- 

 ened aperture as an occasional character of this shell, 



