LYMN.ECEA. 1 9 



above and below ; body volution rounded on the sides, large ; 

 soire consisting of two or three small, compact, rounded volu- 

 tions; whole shell with nearly obsolete lines of growth, and 

 of a brovv-nish horn colour ; aperture sublunated, or nearly 

 circular. 



Discovered by Joshua Alder, Esq., Newcastle, in a pond on 

 Holy Island, and has also been met with by him and the Rev. 

 W. Mark, at Whitby, Northumberland; and has been found at 

 the Falls, and Blackwater, and Lagar, near Belfast, by William 

 Thompson, Esq. 



4. Planorbis imbricatus, pi. Ill, f. 40, 41. 



Planorhis imhricatus, Miiller, Verm., II, p. 165; Draper- 

 naud, p. 44, pi. 1, f. 49, 50, 51 ; Pfeiffer, p. 84, pi. 3, f. 15; 

 Brard, p. 163, pi. 6, f. 10, 11 ; Brown, Illust. Conch., p. 31, pi. 

 14, f. 46, 47, and pi. 18, f. 11 ; lb.. First Ed., pi. 41, f. 46, 47; 

 Jeffreys, Linn. Tr., XVI, p. 388 ; Alder, Mag. Zool. and Bot., 

 II, p. 114; Tmton, Man., p. 11, f. 95 ; Forbes, Mai. Mon., p. 

 13; Thompson, Ann. and Mag. Nat. Hist., VI, p. 49; Planor- 

 his nautileus, Fleming, Edin. Ency., VII, p. 69 ; lb., Brit. An,, 

 p. 278 ; Turbo nautileus, Brown, Ency. Brit., VI, p. 457 ; lb., 

 Wernerian Mem., II, p. 522 ; Heli.v nautileus, Walker, f. 20, 

 21 ; Montagu, Test. Brit., p. 464, pi. 25, f. 5. 



Variety 1, pi. Ill, f. 42. Shell consideraUy smaller, tvith 

 the imbricated lamince more distant. 



Planorbis cristatus, Drapernaud, p. 44, pi. 2, f. 1, 2, 3; 

 Pfeiffer, p. 84; Alder, Mag. Zool. and Bot., p. 114; Brown, 

 Illust. Conch., p. 31, pi. 18, f. 10. 



Variety 2. With the laminae quite obsolete. 



Shell considerably depressed, pellucid, of a greenish or black- 

 ish horn colour; sides of the body rounded; spire consisting of 

 two or three volutions, well divided by the suture line ; some- 

 what largely umbilicate above and below ; body volution pro- 

 vided with transverse spinous ribs, forming a regular crest round 

 the centre of the sides ; aperture slightly ovate ; outer lip con- 

 tinuous all round. Diameter the tenth of an inch. 



Not unfrequent in ponds and ditches, adhering to aquatic 

 plants, in many parts of Great Britain and Ireland. Monstro- 

 sities of this species, with the volutions detached and raised 

 above each other, were found by Dr. Turton in England, and 

 in Ireland by William Thompson, Esq., of Belfast. 



