258 PROCEEDINGS OF THE MALACOLOGICAL SOCIETT. 



in captivity proved inconstant. Temporary surroundingf?, again, affect 

 the colour of slugs, a result which has been said to be due to slime 

 secretion. Nudibranchs will vary with their habitat, and their 

 brilliant colouring will become somewhat faded in captivity. 



Planorbis corneus has been observed with a flesh-coloured or pink 

 animal. 



The shell is, as is well known, usually more brightly coloured in 

 land molluscs, in warmer climates ; this is also noticeable amongst 

 marine shells, Arctic species, as also those from the deep sea, being 

 usually inconspicuous in colour. In dry and arid regions the shell is 

 often white and thick, the thickness hindering evaporation, and the 

 whiteness being a protection from the sun's heat. Variation of food 

 will alter the colour of the shell, and a curious brown- coloured race of 

 Helix hortensis has been found ^ near tanneries, while yellowish forms 

 of H. nemoralis are said^ to prevail in dried grass, red or reddish- 

 brown being most common at the edge of woods. In America it is 

 stated ^ that the colour of Pyramidula alternata, which lives in 

 decaying wood, varies with the kind of tree in which it is found, and 

 resembles that of the surroundings ; this latter consideration, however, 

 is foreign to my purpose at the moment. Similarly a damp season 

 produces an effect. ' 



Mr. Brindley^ recently found that of 639 H. arhusforum 465 were 

 adult, and of these 15 were without any trace of banding, while of 

 261 H. nemoralis 21 were bandless, and these were mostly brownish 

 in colour. 



Rays of colour in the Unionidse are most commonly found ^ in 

 specimens from running, not still water ; while on the point of colour 

 rays or bands in general I may remark that they are exceedingly 

 inconstant, and may arise or cease suddenly. 



The entire absence of any colouring in shells which normally are 

 coloured is by no means rare. This is not, in general, correlated with 

 any apparent change in the animal, and cases of albinism in the 

 animal are very scarce. Vivipara vivipara has been seen ^ in an isolated 

 example to be absolutely lacking in black pigment, even in the eyes, 

 and the same has been recorded in Jgriolimax. 



Shells may be white, or transparent and colourless. The causes may 

 be various ; food, a damp season, or some inherent defect in the 

 secreting power of the animal have all been stated to cause it, and it 

 may occur either in isolated specimens or in whole colonies. In France 

 albinism is said to occur more in the north and in mountainous regions. 

 White Helix incarnata are said to have had white descendants, but in 

 other cases normal young have been bred. Gredler suggested that the 

 white forms were perhaps hereditary, but somewhat morbid, and he 



' Martens: Nachrbl. Deutsch. Malak. Ges., 1872, p. 44. 



2 Winkworth: Essex Nat., vol. xiii, p. 256. 



^ Ormsbee : Nautilus, vol. x, p. 64. 



■• Mollusca in Nat. Hist. Cambridgeshire, p. 122. 



* Jordan: Biol. Centralbl., vol. i, p. 392. 



^ Simroth: Zool. Anz., vol. ix, p. 403. 



