ii6 



Proceedings of the Royal Irish Academy. 



specimens of this shell run very close 

 to the extreme forms of Planorhis spir- 



WDED LD AN 



FE TY AR DO 



WMSL LE MO 

 EM RO CV LH 



WG NG LF WH ME 

 SG KG i^D DU 



GL NT QO ow wi 

 NK LK ST KK WX 



SK MC EC WA 



wc 



orhis var. leucostoma ; but the latter is 

 mainly a marsh shell ; whereas the pre- 

 sent one, like Planorhis carinafus, is 

 seldom met with except in rivers, 

 canals, or lakes. 



Planorbis spirorbis (Linue). 



Helix Spirorbis, Brown, Mem. Werner. 

 Soc, vol. ii, p. 524, 1818. 



WD ED LD AN 

 FE TY AR DO 

 WMSL LE MO 



EM RO CV LH 

 WD NG LF WH ME 

 SG KG KD DU 

 GL NT QG GW WI 

 NK LK ST KK WX 

 SK MG EG WA 

 WG 



Generally distributed, but rare in 

 some of the central counties, where it 

 is to a certain extent replaced by Plan- 

 orbis vortex. Two well-marked forms, 

 which are defined below, are to be found, 

 the extreme examples of which might 



easily be given specific rank ; but it is 

 difficult to distinguish the intermediate 

 forms which seemingly connect these 

 together. 



(rt) This is often regarded on the 

 Continent as the type of the species. 

 It has more quickly increasing whorls, 

 and in some characters resembles the 

 northern form of Planorbis glaber. 

 Mainly confined to the southei'n counties ; 

 but specimens from Dog's Bay, in West 

 Galway, have been referred by the late 

 Prof. Boettger to the " typical Conti- 

 nental P. spirorbis." 



(b) The Planorbis leucostoma of 

 Michaud. Distinguished by the pos- 

 session of an internal white rib round 

 the aperture, flatter shell, and more 

 slowly increasing whorls. This is the 

 prevailing form, and is particularly 

 common in the west, and occurs on 

 most of the western islands. Large 

 and specially flat specimens of this 

 form are often confounded with Plan- 

 orbis vortex. 



Some specimens of tliis last variety I 

 cannot separate from Galioian examples 

 of Planorbis septemyyratus llossm., from 

 the Westerluud collection in the 

 National Museum, Dublin. 



Planorbis contortus (Linne). 



Selix eontorta, Brown, Mem. Werner. 

 Soc, vol. ii, p. 524, 1818. 



Common and generally distributed 

 over most parts of the mainland, but 

 rare in many of the western districts ; 

 and the only island on which it has 

 been taken is EathUn, in Antrim. It 

 is a shell which varies greatly in size, 

 probably according to the nature of the 



