BARDWICKR'S SCIENCE-GOSSIP. 



77 



generation would have perfect spires, the decollation 

 not being transmitted. 



White varieties would seem to be caused by the 

 non-development of the colour-forming organ, but, 

 again, as in the cases given above, white varieties 

 are also local, being confined to one spot, or to one 

 neighbourhood, and it is rarely that, one having been 

 found, a careful search does not reveal others. That 

 these white or colourless varieties are due to the 

 nature of the food or of the soil is unlikely, because 

 they are always, or nearly always, found with 

 typical coloured specimens. 



It cannot be due to a contagious disease of the 

 colour-gland, as in that case we should find specimens 

 which had commenced life with coloured shells, but 

 having subsequently lost the colour-forming function, 

 would have the last few whorls colourless ; and I 



fig- 55- — Helix ltevij>es, var. 

 alba. Calcutta. A nor- 

 mally sinistral Helix. 



Fig- S7-~ Limnaa stagnalis, 

 monst. scalariforme. 

 Chislehurst Common. 



Fie. '56. — Stetwgyra decollata- 

 Marocco. An instance, of 

 normal decollation. 



Fig. 53. — Valvata piscinalis, 

 depressed variety. Crayford 

 brick earth. 



Fig- 59- — Physa fontinalis. 

 A large specimen. Ealing. 



Fig 



60. — Physa acuta. 

 Kew Gardens. 



have never seen or heard of such a specimen. 

 Again, there are the colour varieties, also local in 

 their distribution, and apparently, though of course 

 not really, without cause. The bright colours of 

 some varieties of Helices, such as H. nemoralis, seem 

 even to be injurious, as they make the shells such 

 conspicuous objects as they crawl, and enable the 

 birds to find them readily ; and, that birds do eat 

 numbers of these brilliantly coloured snails, is well 

 testified by the heaps of broken shells round a suitable 

 breaking-stone. But there is one instance, that of 

 the green H. nemoralis at Crayford (see page 236), 

 in which variation would appear to be protective, 

 but it was a variation in the animal, and not in the 

 shell, that caused the green tint. 



And there are many other points which seem to me 

 to need careful study before any conclusions can be 

 arrived at, and I will give an instance : of what use 

 are the bands to the helices ? why are they developed ? 



and why do they vary so much ? All I can say is 

 that I do not know why, but it would seem that 

 form from which the now existing helices were 

 developed had five definite bands, like H. nemoralis ; 

 or perhaps, we may go still farther back and say that 

 the form from which all the Gasteropoda sprung, the 

 first type of the Gasteropod shell-bearing Mollusc, 

 was banded. The reason for this speculation is that 

 the bands are always in the same relative position in 

 the Gasteropoda when they are developed, the band 

 just above the periphery being specially characteristic. 

 However, this is a subject to which little attention 

 seems to have been given, but I think that it will 

 well repay research. 



In the present paper I shall not deal with so huge 

 a subject as the variation of the mollusca throughout 

 the world ; I leave this to others, and shall only 

 describe the variation of the mollusca in the counties 

 of Kent, Surrey, and Middlesex, the counties which I 

 am now working. 



Gasteropoda. 



Neritina fluviatilis. — This shell does not seem to 

 vary much, and I have never taken an abnormal 

 form. Nevertheless four varieties have been recorded 

 as British.* 



Paludina vivipara. — The bandless form (var. 

 efasciata, =v3.r. unicolor), has been recorded from 

 Richmond, I have not taken it myself. 



Bythinia tentaculata. —The colourless or white 

 variety occurs in the district, and the varieties 

 ventricosa and excavata are also recorded. Monst. 

 decollatnm I have found at Barnes. 



B. Leachii. — Var. elongata is recorded for West Kent. 



Valvata piseinalis. — I have taken a variety showing 

 traces of bands ; var. subcylindrica, a dead shell at 

 Hammersmith. The type and a variety approaching 

 var. depressa occur fossil at Crayford. 



Planorbis lineatus, var. albina has been recorded 

 for East Kent. 



Planorbis nautilens. — This has two main forms, 

 the so-called type and the var. crista. Both are 

 found in the district. 



Planorbis spirorbis. — A dead shell of var. albida 

 at Bedford Park (D. B. Cockerell). 



P. vortex. — At Fulham I have taken a variety 

 of this species. (See p. 14.) 



P. carinatus. — I have found the variety disciformis 

 near Guildford. 



P. complanatus. — Mr. J. W. Taylor describes the 

 monst. sinistrorsam from a specimen found by Miss 

 Hele at Wye. This instance of a sinistral monstrosity 

 of Plano7-bis is, I believe, unique. The white variety 

 is said to have been taken in West Kent, and also 

 the var. rhombea. 



P. corneus. — This species varies in size, the largest 

 specimen I have found is from Ealing. The young 



* Mr. R. A. Freeman has a specimen having a. broad white 

 band below the periphery which he found near Barnes. 



