324 



SCIENCE-GOSSIP. 



of mud on them. The coating in Vahata piscinalis, 

 however, may be very thick and firmly adherent. 

 They are thus practically impossible to distinguish 

 with any certainty by the eye. 



Whether the shell is incrusted or not is largely a 

 matter of locality. In Oxfordshire Buliminus tenta- 

 culata " almost invariably covers its shell with an 

 earthy incrustation " ( le ), while in Herefordshire it 

 is almost as invariably clean. Paludina vivipara is 

 " incrusted with an earthy coating, upon which 

 zoophytes and algse often grow. I have seen it 

 entirely covered with a profuse growth of Batra. 

 chospermum moniliforme " ( 1K ). It has been recorded 

 that this species lose most of the " thick incrusta- 

 tion " on their shells after a few weeks in clean 

 water in an aquarium ( 1£l ), an observation which I 

 can, to a certain extent, confirm. 



In July, 1896, I collected seme Paludina, Planorbis 

 corneiis, etc., in a small branch of the Thames in 

 Berkshire, from among a thick growth of weeds, 

 and they were in many cases only slightly incrus- 

 tated with mud and weed growth. The same 

 stream was then very thoroughly cleaned out, and 

 the specimens from just the same places in June, 

 1897, were conspicuously more covered up and 

 concealed with adventitious matter in their now 

 almost weedless and open habitat. 



Finally, before going on to the variation of our 

 mollusca, we may briefly consider what is known 

 generally as to the nature of these colours. Con- 

 trary to a statement which is sometimes made ( 20 ), 

 the colouring matter, as a rule, in British 

 Gastropoda resides in the main in the superficial 

 part of the calcareous part of the shell. This 

 may be easily demonstrated by peeling off the 

 periostracum (with the help of a brief application 

 of fairly strong acid), when the pigmentation will 

 still be seen on the freshly exposed surface. But 

 it is considerably modified by the periostracum : 

 e.g., the difference in colour between Tachea 

 nemoralis v. castanea and T. hortensis v. lilacina 

 has been ascribed solely to differences in the 

 nature and tint of the periostracum, which is 

 thicker and browner in the former species ; and 

 there is no doubt that deperiostracization does 

 bring the tints of the two shells much nearer to 

 one another, castanea becoming much more violet 

 in tint. Again, the violet-purple tint sometimes 

 seen on deperiostracized portions of dead and empty 

 shells of Cryptomphalus, as contrasting with the 

 brownish colour of the parts which are still covered 

 with the periostracum, is familiar to most collectors. 

 Going still further, we find that in some species 

 the colouring matter resides entirely in the perio- 

 stracum (e.g., Clausilia laminata, Acanthimda aculeata). 



(&) J. F. Whiteaves, op. cit. p. 15. 



( 19 J R. M. Christy : " Zoologist " (3) v. (1881), p. 181. 



I 20 ) e.g., A. M. Marshall and C. H. Hurst : " Practical 

 Zoology," ed. 2 (1688), p. 107. W.Williams: "Shell- 



Collectors' Handbook " (1888), p. 



This seems mainly the case with most horn- 

 coloured species ; but some of these, at any rate, 

 have a slight brownish tinge in the calcareous matter 

 (some Hyalinia, Amphibulima = Succinea). With 

 regard to the aquatic species, Mr. Bowell says : 

 " Really the species which come under this 

 designation (horn-coloured) should be classed as 

 colourless. Bythinia tentaculata, for instance, varies 

 from brownish horn-colour to clear white, the 

 difference being solely due to the presence or 

 absence of a sufficient amount of adventitious 

 matter. From Burnham, in Somerset, I have 

 some white forms. They do not really represent 

 albinism." 



While admitting that the tint of the shell may 

 be thus somewhat modified (that is after a good 

 external cleaning), I think this is an extreme 

 view. The separated ( 21 ) periostracum of B. tenta- 

 culata is brown, and less brown than the intact 

 shell. Further, albine and brown specimens are 

 found together, and the bleaching action of light 

 acts rather readily on this species, as seen in the 

 museums, which is hardly what we should expect 

 if the colouring were entirely "adventitious" in 

 nature. In the Pelecypoda it is the periostracum 

 which contains the pigmentation. 



Whence comes this pigment in the shell ? 

 Everyone is familiar with the black or dark -brown 

 patches or bands on the mantle of Tachea corre- 

 sponding in position with the bands on the shell, 

 and with the densely but patchily pigmented 

 mantle of Cryptomphalus, with its dark and light 

 patched shell. The suggestion contained in this is 

 probably correct : viz., that the colouring of the 

 shell has something to do with the pigment in the 

 mantle. On the other hand, Limnaea is often quite 

 thickly pigmented, but still has a plain shell. 

 Examined microscopically, the pigmentation of the 

 mantle in Tachea is seen to be due to an accumulation 

 of pigment cells of a squarish or oblong (but 

 irregular) outline, filled thickly with globules, 

 which, seen singly, appear rather pale- brown 

 when examined fresh. In Limnaea the pigment 

 cells are of the more usual stellate, branched type, 

 and contain apparently granules rather than 

 globules, the pigment being more amorphous. 

 This pigment has been studied in Limnaea by 

 E. Andre (— ). He finds that it is not melanin, as it 

 is usually called, but an iron-containing substance 

 with similar properties to the melaine of the ink- 

 bag of Cephalopoda. It differs from the group of 

 vertebrate pigments known as " melanin " in some 

 of its reactions and solubilities. 



It is noticeable that in newly-hatched Paludina 

 the mantle becomes pigmented, while the shell 



( 21 ) Periostracal " casts " of this species are not hard to 

 prepare with dilute acid. 



( 22 ) Revue SuisseZool., iii. (1895), p. 429. Journ. Roy. Micr. 

 Soc, 1896, p. 303. "Melanin" seems at best to be only a 

 generic term. 





