Ill VARIATION IN SNAILS 8/ 



whole to increase, although there are certain genera (e.g. Pecten} 

 which are not more brightly coloured in Mediterranean than in 

 Icelandic waters. 



Land Mollusca inhabiting the mainland of a continent not 

 imfrequently become smaller when they have spread to adjacent 

 islands where perhaps the rainfall is less abundant or the soil 

 and food-supply less nicely adjusted to their wants. Orthalicus 

 undatus is decidedly larger on the mainland of S. America than 

 on the adjacent islands of Trinidad and Grenada. Specimens 

 of Bulimulus exilis from Barbados are invariably broader and 

 more obese than those from S. Thomas, while those from the 

 volcanic island of S. Lucia, where lime is deficient, are small and 

 very slender. Streptaxis deformis, as occurring at Trinidad, is 

 only half the size of specimens from Georgetown, Demerara.^ 



Certain localities appear, for some unexplained reason, to be 

 particularly favourable to the production of albino varieties. The 

 neighbourhood of Lewes, in Sussex, has produced no fewer than 

 fourteen of these forms of land Mollusca and five of fresh water.- 



Om" common Helix aspcrsa, as found near Bristol, is said to 

 be ' dark coloured ' ; about Weston-super-mare brown, with black 

 markings ' ; near Bath ' very pale and much mottled ' ; at Cheddar 

 ' very solid and large.' ^ Sometimes the same kind of variation 

 is exhibited by different species in the same locality. Thus 

 specimens of H. aspersa, H. nemoralis, and H. hortensis, taken 

 from the same bank at Torquay, presented a straw-coloured tinge 

 of ground colour, with red-brown bands or markings. Trochi- 

 form H. nemoralis and H. arlustoTum, sinistral //. hortensis and 

 H. aspersa, sinistral H. asinrsa and H. virgata, and similarly 

 banded forms of H. caper ata and H. virgata, have been taken 

 together.^ 



The immediate neighbourhood of the sea appears frequently 

 to have the effect of dwarfing land Mollusca. Thus the var. 

 conoidea of Helix aspersa, which is small, conical, with a com- 

 pressed mouth, occurs 'on sandhills and cliffs at the seaside.' 

 The varieties conica and na7ia of Helix hispida are found ' near 

 the sea.' Helix virgata is exceedingly small in similar localities, 

 and tends to become unicoloured. H caperata var. Gigaxii, a 



^ J. S. Gibbons, Journ. of Conch, ii. p. 129. 



* C. II. iloiris, ibid. vii. p. 191. » F. M. Hele, ibid. iv. p. 93. 



■* T. D. A. Coukerell, Science Gossi]), 1887, p. 67. 



