Ii6 



SCIENCE-GOSSIP. 



on June 26th I collected a few Neritina fluviatilis 

 from a rather quickly-flowing ditch near Oxford, 

 and placed them in a small still pond here ; on 

 August 2nd the only one I could find was very 

 extensively, though superficially, eroded, all the 

 spotted colouration being removed. The species is 

 notoriously adverse to quite still water, and difficult 

 to keep alive during confinement. 



When a decalcified section of the shell of a bi- 

 valve (1) is examined, the appearance presented 

 immediately beneath the two layers of periostracum 

 is of a number of irregularly angulated tubes with 

 very thin walls, lying perpendicular to the surface 

 of the shell, and it looks as if each prism of CaCOs 

 in the outer layer were encased in a thin envelope 

 of organic matter. And there is nothing which 

 makes it impossible that these slender net-works 

 should not permeate the whole shell — despite the 

 denser construction of the nacreous layer — and join 

 the periostracum to the adductor muscle. Hence, 

 it will be seen how erosion may follow ill-health, 

 and it is a matter of common observation that 

 erosion often occurs in waters which are seriously 

 contaminated with some impurity. At the same 

 time I think that one other cause at least may act 

 directly in removing the periostracum : the brilliant 

 French researches referred to above have shown that 

 various algae bore through the periostracum and 

 right into the calcareous part of the shell in many 

 marine and freshwater mollusca ; and the holes thus 

 made may eventually lead to extensive erosion. 



Finally, to turn for a short time to terrestrial 

 species : one of the most interesting forms of erosion 

 here found is that consisting of spiral bands or 

 grooves. This phenomenon is often seen in Tachea, 

 and is very frequently post-mortem in character. 

 Mr. T. D. A. Cockerell C^) drew attention to it some 

 time ago, and stated that similar results could be 

 obtained by treating the shell with dilute hydro- 

 chloric acid. He failed to thus produce them in 

 Tachea nemorahs, nor had he observed them in 

 naturally -weathered shells of that species. They 

 occur, however, in both T.nemoralis andT. hortensis. 

 The bands arise, no doubt, from the fact that some 

 spot in the secreting edge of the mantle, makes, so 

 to speak, a periostracum of inferior quality, and 

 that when the snail dies, the weathering action of 

 the rain, etc., first removes this weak tract, which, 

 from the nature of the case, follows a course " cor- 

 responding to the position of the bands on other 

 shells." These spiral bands, due either to erosion 

 or to some distortion («), occur in many species (^). 



The case of normally decollate species is very 

 curious : Bulimus decollatus is, of course, the regular 



(1) I have chiefly used M. margarilifera. 



(2j " Zoologist " 1885, p. 114. 



(3) Probably of the prismatic layer. 



(*) A list is given by Mr. S. C. Cockerell in Journ. of 

 Conch., iv., p. 374, to which other species might be added. 



example. Some genera, too (e.g., CylindriUaJ, are 

 normally decollate. A sort of parallel may perhaps 

 be found in those freshwater species {e.g., Limnaea 

 glabra) which are so very frequently badly eroded. 

 Here the conclusion is forced upon us that the 

 phenomenon must be in some way hereditary, 

 though how it took its first origin is not very clear. 

 The explanation that the upper whorls become 

 disused, vacated and then absorbed, seems to have 

 much to recommend it : if this view is correct, the 

 case is not exactly parallel with ordinary erosion. 

 Mr, B. Tomlin (») has recorded some very interesting 

 observations on Clausilia : he notices that those 

 species which live in old walls, etc., get the project- 

 ing parts of their shells weathered, eroded and 

 decollated {e.g., C. riigosa, itala, parvida and plicata), 

 whilst such species as C. rolphii, which is essen- 

 tially geophilous, and lives among dead leaves, etc., 

 are never decollated. A similar principle might 

 apply to man 3' elongated forms. 



The Grange, Hereford ; August 24th. 1896. 



DEATH'S-HEAD MOTHS. 



A S all the older lepidopterists know so well, 

 -^~^ nearly every year is celebrated for the 

 exceptional occurrence of some particular butterfly 

 or moth. This season seems likely to be known in 

 England as a " death's-head " year, for we hear of 

 its having been taken in exceptional numbers in 

 the caterpillar state throughout England and 

 Ireland. We regret that we cannot find space in 

 this number for all the records which have been 

 sent to us, but the following extract from a letter, 

 dated September 15th, received from Mr. Herbert 

 A. Hole, of Harbury, Harcourt Road, Newark, 

 may be considered typical: "During the last 

 six weeks, both the larvse and pupae of Acheroniia 

 atropos appear to have been exceptionally abundant 

 in the town of Newark-on-Trent, and in one or two 

 of the neighbouring villages. Unfortunately my 

 business is such that it prohibits my making a 

 personal search during the best hours of the day. 

 Most of my workmen, however, have allotments, 

 chiefly devoted to the culture of potatoes, and it is 

 to their goodness, knowing I take an interest in 

 entomology, that I am indebted for at least ten 

 specimens of the chrysalis and caterpillars ; several 

 others have been brought to me for identification. 

 Although for about seven years I have worked this 

 immediate neighbourhood, I have never come across 

 either the perfect insect or larva, This particular 

 season it seems to be comparatively in abundance. 

 Will anyone tell me in tjie next issue of your 

 interesting paper if this is an unusual incident ? " 



Those persons who possess pupas will do well to 

 force them at once with artificial heat. 



(=) Brit. Nat. i , pp. 227-229. . 



