FAMILY LYMN^ACEA. 145 



minutely speckled with black, and are in all respects similar. When 

 the animal of P. vortex retires into its shell in a torpid state from 

 lack of moisture while the shallow pools which it inhabits are dried 

 up, it forms a callous ridge within the margin of the aperture. 



Mr. Macgillivray, who carefully observed the habits of this species 

 from specimens collected in the neighbourhood of Aberdeen, re- 

 marks that the animal slowly glides along the surface of a stem 

 or leaf under water by a series of undulations. Every now and 

 then the shell is jerked forward by a sudden movement, which is 

 not performed by the foot, but by the muscles which pass from it, 

 or along the neck into the body. It is abundantly diffused through- 

 out the British Isles, and in all parts of Europe, passing into Siberia 

 and North Africa. 



8. Planorbis spirorbis. Roll Planorbis. 



Shell ; extremely depressed, dingy horny, thin, upper and lower 

 disk nearly equally concave, each showing impressed 

 sutures ; whorls five to six, finely striated, convex, r -^—& 

 slightly angled at the margin of both disks ; aper- 

 ture lunar ovate, mostly ridged within. 

 Helix spirorbis, Linnaeus (1758), Syst. Nat. 10th edit. p. 770. 

 Planorbis spirorbis, Miiller (1774), Verm. Hist, part ii. p. 161. 

 Planorbis rotimdatus, Poiret (1801), Prod. p. 93. 

 Planorbis leucostoma, Millet (1813), Moll. Maine et Loire y 



p. 16. 



Planorbis septemgyratus, Ziegler (1835), Rossm. Icon. vol. i. p. 103. f. 64-. 

 Planorbis Pereeii, Graells (1850), Dupuy, Hist. Moll. vol. iv. p. 441. pi. 



xxv. f. 6. 

 Planorbis frag His, Millet (1854), Moll. Maine et Loire, 3rd edit. p. 43. 

 Planorbis (GyrorbisJ spirorbis and rotimdatus, Moquin-Tandon (1855), 

 Hist. Moll. vol. ii. p. 435 and 437. pi. xxx. f. 38 to 46 and pi. xxxi. 

 f. 1 to 5. 

 Hab. Throughout Europe. Siberia. North Africa. (On plants in shal- 

 low stagnant water.) 

 This species is very closely allied to the preceding, and it has a 

 similar distribution. Both were, however, separately distinguished 

 by Linnaeus and Miiller, and they have been regarded as separate 

 species by all subsequent writers on the subject. Typical speci- 

 mens of P. spirorbis are composed of fewer whorls than P. vortex, 



L 



