242 LIMN^AD^. 



109. 7. Planoebis vortex. Whorl Coil Shell. — • 

 Shell brown, pellucid, thin, flat above, slightly 

 and regularly concave beneath, v/ith six or 

 seven gradually increasing sharply keeled 

 volutions, which are convex before and flat 

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

 f. 91.) 



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



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



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



Hist.v'i. 154.; Jeffreys, Linn. Trans, xvi. 382.; Bossm. Icon. 



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



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



B. M. iv. 157. t. 127. f. 6, 7, 8. — Helix i)lanorbis. Da 



Costa, p. 65. t. 4. f. 12. — Planorbis conipressus. Michaud, 



Conipl. 81. t. 16. f. 6. 8. — Planorbis tenellus. Studer, 



Cat. 92. 

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



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



JRossm. Icon. i. 105. f. 62. — Planorbis rotundatus. Pair. 



Prod. 93.; Moq. Tand. ii. 435. t. 30. f. 38— 46. — Planorbis 



vortex l3. Drap. 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, Avith 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, wlien the ditches are dry, this animal 

 closes up its shell with a white epiphragm, Avithin 

 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 4nternal 

 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. 



