144 BRITISH MOLLUSKS. 



is not the most appropriate, for the species is the less smooth of the 

 two, and it is darker in colour. It is much the commoner, being 

 present in all parts of Britain, excepting Scotland, in stagnant 

 marshes, ponds, canals, and ditches. Dr. Gray remarks that P. 

 complanatus breeds very rapidly in ponds of warm water that is 

 emitted from steam-engines in Yorkshire ; and the specimens found 

 in such situations have a curious tendency to assume a more spiral 

 form. The range of P. complanatus abroad is similar to that of 

 P. carinatus, and the two species are very closely connected by an 

 intermediate form, P. submarginatus of Cristofori and Jan, P. in- 

 termedins of Charpentier. 



7. Planorbis vortex. Wliirl Planorbis. 



Shell ; extremely depressed, fulvous horny, thin, upper disk mo- 

 derately concave with the sutures rather in- 

 dented, lower disk flatly concave, with the whorls d==5^ 

 sloping to the lower margin, where they are ob- 

 tusely angled ; whorls six to seven, very nar- 

 row, increasing very slowly, faintly obliquely 

 striated ; aperture obliquely lunar, lip simple. 



Helix vortex, Linnseus (1758), Syst. Nat. 10th edit. p. 772. / \ K, .■; 



Planorbis vortex, Miiller (1744), Hist. Verm, part 2. p. 158. i : / 



Helix planorbis, Da Costa (1778), Test. Brit. p. 65. pi. iv. '>,~~~}..f 

 f. 12 (not of Linnaeus). 



Planorbis tenellus, Stucler (1820), Kwrz. Verz. p. 92. 



Planorbis compressus, Michaud (1831), Compl. p. 81. pi. xvi. f. 6 to 8. 



Planorbis (G-yrorbis) vortex, Moquin-Tandon (1855), Hist. Moll. vol. ii. 

 p. 433. pi. xxx. f. 34 to 37. 



Hob. Throughout Europe. Siberia. North Africa. (Ou plants in shal- 

 low stagnant water.) 



P. vortex and spirorbis are very similar, and shoidd also be studied 

 together. Their shells are of all land shells the most thinly, slowly, 

 and flatly coiled. That of P. vortex is composed of from six to 

 seven whorls, sloping towards the lower margin, where it is obtusely 

 keeled, and the lower disk of the shell is consequently less concave 

 than the upper. In P. spirorbis the shell is smaller, and has from 

 one to two fewer whorls, the whorls are less angled at the peri- 

 phery, and more truly discoid, so that the two disks are nearly equally 

 concave. In both species the animals are purplish reddish-brown, 



