DISTRIBUTION OF ANIMALS AND PLAN IS IN TELSTEAD, KSSKX. 195 



adapting themselves to some untoward circumstances which would 

 mean death to other organisms, the very general restriction of certain 

 forms to ponds, and others to swift streams, leaves no other solution 

 than that the stillness or temperature of the water is the important 

 factor, (iiven, therefore, the river basin and the low-lying lands, 

 another period of lakes and morasses, and perhaps some increase of 

 woodland to assist in the preservation of moisture, there would seem 

 to be nothing to hinder the universal distribution of all the forms 

 now peculiar to their several localities. 



Taking now the distribution of the land moUusca, we are assisted 

 by a historical data which appertain sparingly to the water molluscs, 

 and not at all to the fish. These are the fossil remains of some shell- 

 marls. In vol. iii., page 14, of The Essex Naturalist, I gave 

 a table of thirteen species from these deposits. Seven of these 

 species are now locally extinct, four are very rare and probably 

 locally extinct, and two, frequent or common in the past, are now 

 rare. The substantial truth of that list I have since verified more 

 particularly, and although the number of species in it may be 

 increased,^ it will not need material alteration. It is very difficult to 

 account for this change among the land molluscs, the more so that 

 two species, now very rare, seem to have been common, say in Roman 

 times or somewhat earlier. These are Helix m-bustorum and 

 Cydostoinn e/ega/is, both having been found in shell-marl connected 

 with an artificial lake (see E. N., vol. vi., page 35). Of the former 

 Mr. Christy wrote me some time ago that in his neighbourhood 

 (Chignal St. James) he was astonished at the former abundance of 

 these shells, and their present rarity, which is also my experience. 

 Yet the conditions, except that the land is better drained, so far as 

 we know, have not altered. Of the latter species {C. elega/is) I have 

 given some account (E. N., vol. iv., page 92). It is true that in 

 some places the I5oulder-clay has wasted, and if it is true that the 

 animal requires a limestone habitat, this fact may, in some measure, 

 account for its disappearance. It would be interesting to know 

 whether Cyclostoma occurs as a fossil m Essex south of the line of 

 Boulder-clay. A peculiarity of its distribution is that, although 

 common on the Chalk hills of Charlton, on the opposite bank of the 

 Thames, it does not now live in South Essex. 



For the disappearance of the thirteen forms mentioned in the 



I Mr. Christy calls attention to Helix aspersa as not having been named, but it appears to be 

 unrepresented in the older marls. 



