268 PNEUMONOBRANCHIATA. 



Miiller long ago pointed out the white and thick- 

 ened aperture as an occasional character of this shell, 

 though he was not perhaps aware of the manner in 

 which it was formed. 



107. 8. PLANORBIS spirorbis. Rolled Coil Shell, 

 (t. 8. f. 98.) Shell thin, brown, slightly concave 

 on both sides, with six gradually increasing round- 

 ish and nearly equal volutions ; mouth roundish. 



Planorbis spirorbis. Miiller, Verm, ii. 161. ; Brard, 

 p. 156.; Turton, Man. ed. 1. 115. f. 98.; Sturm, 

 Fauna, t. 45. 



Helix spirorbis. Gmel. i. 362. ; Mont. p. 455. t. 

 25. f. 2. 



Planorbis vortex j3. Drap. Moll 45. t. 2. f. 6, 7.; 

 Jeffreys, Linn. Trans, xvi. 382. 



In ponds and canals. 



Shell in flatness and compactness of volutions much 

 resembling the P. vortex, but is less and has only six 

 volutions ; 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. spir- 

 orbis of Miiller are distinct. (See Alder, Cat. Supp. 2.) 



The peristome is often white-ribbed. 



* * * Shell polished ; spire deeply umbilicate ; whorls 

 slightly keeled ; front of shell convex. 



108. 9. PLANORBIS nitidus. Fountain Coil Shell, 

 (t. 8. f. 93 ) Shell depressed, dark horn-colour, 

 smooth, glossy, diaphanous, flat above, with a cen- 



