88 THE LOCAL EXTINCTION OF MOLLUSCS. 



the attention of Essex naturalists for many years ; I allude to 

 the almost complete extinction of Cyclostonia elegans, compared 

 with its ^reat abundance in Roman and comparatively-recent 

 times. This shell was thought to be now quite extinct in Essex, 

 but in the year i8go I found a small colony of them alive in 

 Felstead, and they are still in existence (see Essex Naturalist, 

 vol. IV., p. 92). Cyclostoina is always, I believe, described as 

 associated with chalk or limestone. In Essex this chalk was 

 supplied by the Boulder Clay, or by the tufa which is formed by 

 the re-deposit of chalk at places where springs issue. But the 

 Boulder Clay is a wasting product, and is only occasionally 

 found on the valley slopes, neither is it continuous over very 

 large spaces on the higher ground,' and where it is, drainage and 

 other artificial agencies have deprived the snail of its shelter and 

 broken up its colonies (always an important step towards local 

 extinction), so that Cyclostoina elegans has now almost completely 

 disappeared. 



Another shell that is generally described as affecting a lime- 

 stone habitat is Helix hpicida. This species is, I feel sure, 

 verging also on total extinction, although still living in some 

 parts of Essex. There are no data, so far as I am aware, to 

 prove that it ever approached Cyclostoma in numbers, but there is 

 sufficient evidence to show that its wide-spread distribution was 

 due to conditions other than those now in operation. The stage 

 of separation and isolation is now very far advanced, and Helix 

 lapicida is always quoted as being local and in very small 

 numbers. Where such a condition of distribution obtains over 

 a very wide area, it is pretty clear that the species was once 

 common in that area. I suggest that the wasting away of cal- 

 careous matter from the surface soil is a probable cause for the 

 disappearance of this species."* Perhaps the greatest factor of all 

 here at work in bringing about local extinction is the drainage of 

 the land ; this affects the Molluscs indirectly in many ways. In 

 former times, when even the uplands were sometimes sour 

 morass, growing mosses in abundance, and the county was well 

 wooded, a certain amount of moisture was stored up in the day- 

 time, to be given out as aqueous vapour and received back as 



3 In reply to objections by Mr. T. V. Holmes and Mr. Whitaker, that "the Boulder Clay 

 is continuous over the greater part of N. Essex and forms the surface of most of the high 

 ground," Mr. French remarked that in " my neighbourhood (Felstead) Boulder Clay is certainly 

 not continuous. A reference to the geological map is misleading, because the map only takes 

 into account the subsoil. Wherever tillage is in progress, the surface soil, which is that in 

 wrhich Molluscs work, is deprived of its calcareous ingredient." — Ed. 



4 The greatly-increased preservation of birds may account for the disappearance of some 

 species of snails. — T. V. H. 



