PLANOKBIS. 243 



though he was not perhaps aware of the manner in 

 which it was formed. 



no. 8. PLANORBIS spirorbis. Rolled Coil Shell. 

 Shell thin, brown, slightly concave on both 

 sides, with six gradually increasing roundish 

 and nearly equal volutions ; mouth roundish, 

 (t. 8. f. 98.) 



Helix spirorbis. Linn. S. N. 1244. ; Mont. T. B. p. 455. t. 25. 

 f. 2. Plan orbis spirorbis. Miiller, Verm. ii. 161.; Brard, 

 p. 156. ; Drap. H. M. ii. t. 2. f. 8, 9. ; Turton, Man. ed. 1. 

 115. f. 98. ; Sturm, Fauna, t. 45.; Leach, Syn. Moll. 112.; 

 Forbes and Hartley, B. M. iv. 159. t. 127. f. 9, 10. 



In ponds and canals. 



Shell in flatness and compactness of volutions 

 much resembling the P. vortex, but is less and has 

 only six volutions, and is rather thicker ; the upper 

 surface is a little concave, in consequence of the 

 volutions being rounded, and the outer one scarcely 

 flattened into a carinated edge ; and the aperture is 

 rounded below. 



Mr. Alder now believes that P. vortex and P. 

 spirorbis of Miiller are distinct. (See Alder, Cat. 

 Supp. 2.) 



The peristome is often white-ribbed. 



d. Shell polished ; spire deeply umbilicate ; whorls 

 slightly heeled ; front of shell convex. (Hippeutis.) 



in. 9. PLANOKBIS fontanus. Fountain Coil Shell. 

 Shell depressed, dark horn-colour, smooth, 

 glossy, diaphanous, flat above, with a central 

 sunk spire, rather convex beneath; whorls 

 three or four, the outer shelving to a keel in the 

 middle; mouth elongate-angular, (t. 8. f. 93.) 



