FAMILY LYMN^ACEA. 141 



4. Planorbis crista. Crest Planorbis. 



Shell ; elegantly discoid, rather depressed, greenish horny, trans- 

 parent, covered by a fine membranaceous epidermis, 

 disposed in transverse ridges which become puck- crs? 



ered, and serrate the periphery of the last whorl, 

 upper disk flattened, with the sutures rather deeply 

 impressed, lower disk excavately umbilicated in 

 the centre, whorls three, slightly angled, a little 

 keeled at the angle ; aperture obliquely rounded, 

 openly expanded. 



Nautilus crista, Linnaeus (1758), Syst. Nat. 10th edit. p. 709. 



Turbo nautileus, Linnaeus (1767), Syst. Nat. 12th edit. p. 1241. 



Planorbis imbricatus, Midler (1774), Verm. Hist, part ii. p. 165. 



Helix nautilea, Walter (1784), Test. Minut. f. 20, 21. 



Planorbis imbricatus and cristatus, Draparnaud (1805), Hist. Moll. p. 44. 

 pi. i. f. 49 to 51, and pi. ii. f. 1 to 3. 



Planorbis nautileus, Fleming (1814), Udin. JEncyc. vol. vii. part 1. p. 69. 



Planorbis (Gyraulus) nautileus, Moquin-Tandon (1855), Hist . Moll. vol. ii. 

 p. 438. pi. xxxi. f. 6 to 11. 



Sab. Throughout Europe. North Africa. (On water-flags in ponds and 

 ditches.) 



The minute semitransparent horny shell of this species, more 

 generally known to collectors by the second name which Linnasus 

 gave to it, is composed of only three whorls, necessarily more ra- 

 pidly enlarging than if it consisted of a coil of a greater number; and 

 the aperture expands like that of a French horn. But the character 

 which chiefly arrests attention is its crested coating of epidermis. 

 At frequent intervals during the growth of the shell it is puckered 

 on the periphery of the outer whorl, and a kind of crest is formed 

 of serrated lashes. The animal as described by M. Moquin- 

 Tandon is greyish-brown, minutely speckled with black, slow in its 

 movements and irritable, carrying its shell a little inclined when 

 crawling. 



It may be observed, on reference to the synonymy, that Linnaeus 

 and Draparnaud both made two species of this. In adult speci- 

 mens, the puckered epidermis is sometimes conspicuous, while the 

 crest of lashes on the periphery is nearly obliterated ; and some- 

 times both are obliterated. The names crista and cristatus have 

 been given to young specimens, and nautileus and imbricatus to 

 adult specimens. 



Planorbis crista is found on water-flags in ponds and ditches in 



