STKES: VARIATION IN EECENT MOLLUSCA. 259 



stated that the albinos were often smaller than the coloured specimens, 

 and found near the boundary of distribution. LimiKBa and similar 

 genera are sometimes found with white colour-bands.' 



In marine shells but little has been recorded, but I may perhaps 

 refer ^ to a curious spot on Portland where, in a marine pond a few 

 yards across, the fauna consisted of two white species, two species of 

 which white varieties occurred without the normal coloured form, and 

 two species where the normal form occurred with white varieties. 



Ere passing from this branch of the subject, I would call your 

 attention to a paper by Cooke ^ in which he discusses the variation of 

 Achatinella multizonata from a geographical standpoint. He states 

 that while the actual specific distribution extends over some little area 

 certain groups of colour-varieties are confined to certain portions of 

 the range, and he concludes that isolation pays a greater part than 

 environment. 



Erosion has been noticed in many species ; specimens of fresh- water 

 bivalves are often eroded near the beaks, where the periostracum 

 is thinnest, and Gastropods often show similar signs, generally on the 

 earlier whorls. It has been attributed * to emission of carbonic acid 

 gas by plants, and also to a deficiency of calcium carbonate in the 

 water; whilst in confinement, Limncsa, Bithynia, and Planorbis have 

 been noticed^ to become eroded and of carious appearance, and the 

 eroded spots have been seen to be occupied by Micrococcus. It has 

 been stated that in a river in North America all the shells below the 

 spot where a pollution by alkaline wastes entered the stream were 

 eroded, while those above were not. In marine shells it has been 

 said ® to be due to boring algae ; and many specimens have been 

 noticed where the animal has thickened its shell internally for 

 protection. 



Decollation, the earlier whorls having been broken off and the 

 cavity closed by a septum, is a normal occurrence in a number of 

 Gastropods, e.g., Trimcatelh , some Cylmdrella, etc. Many long-spired 

 shells do not use the upper whorls, and this part loses vitality, becomes 

 brittle, and breaks off. It may, however, occur abnormally owing to 

 an accident to the spire. In confinement a scalaiiform Eelix aspersa 

 has been noticed," which on more than one occasion formed internal 

 septa, the upper portion breaking off. 



It must be remembered that MoUusca have the power of absorbing 

 the upper whorls, so as to make room for themselves, and further, that 

 they can secrete shelly matter to repair a hole broken in the shell. 

 Helix potnatia can repair the upper whorls ; and, m captivity, Limncea 

 elodes has been observed to repair a hole in the shell in six weeks. 



1 Overton: Joum. Malac, ix, p. 64. 



- Journ. Conch., vol. vi, p. 166. 



3 Pap. Bernice P. B. Mus., vol. ii, pp. 65-76. 



* Beauchamp : Conch. Exch., vol. i, p. 49. Shrubsole : Jouru. Conch., vol. v 



p. 66. ' 



5 Noll: Zool. Garten, vol. xxiii, p. 157. 



* Hensraan : Irish Nat., vol. iv, p. 137. 



' Lataste: Journ. Couchyl,, vol. xxiv, p. 242. 



