1871.] F. StoHczlsa — Notes on terrestrial Mollusca. 145 



tracted isolation admitted of the development of certain persistent 

 peculiarities of the animals in different localities. With regard to 

 the first point, it is a noteworthy fact that most of the cretaceous 

 species of Helicidce, and a large number of the older tertiary ones, 

 belong to the Angystoma group which is so largely represented in 

 this part of the Malayan province. With regard to the second 

 point, it must be remembered that the nature of the ground is a most 

 important agent in the development and prosperity of the molluscous 

 (and any other) fauna (or flora) ; it regulates to a very large extent 

 the geographical distribution of the species. It is well known that 

 limestone ground is more suitable to the existence of land-shells, 

 than any other kind of rock. In the neighbourhood of Moulmein 

 this is strikingly apparent ; for while some of the limestone locali- 

 ties literally swarm with shells, there are barely any to be found 

 on the neighbouring sandstone or metamorphic hills, which in 

 other respects possess a perfectly similar climate, &c. 



The distinction by no means only applies to the number of speci- 

 mens, but it affects equally markedly the habitat of certain species, 

 and even genera. Thus, for instance, there is scarcely a single 

 specimen of a Plectopylis achatina to be met with on a sandstone 

 hill, while at every limestone rock the species may be collected in 

 thousands. The same applies to Plectopylis cyclaspis, though this 

 species is not so common as the former. All the known species of 

 Sophina, nearly all the Sesarce, several peculiar Streptaxis, Rapliau- 

 lus, Pollicaria and others only occur on limestone ground, while 

 Rotula anceps, Helix \_Fruticicola] similaris i and chiefly also the 

 species of Heliearion, occur on sandstone hills and low land. Again 

 some species, like Macrochlamys honesta, Conulema infula (var. attygia), 

 Mycrocystis molecula are to be met with almost everywhere, but speci- 

 mens occurring on limestone localities always possess a more solid 

 and thicker shell, than those on sandstone, or on alluvial ground. 

 The knowledge of the nature of the ground is, therefore, a very im- 

 portant consideration in discriminating species and mere varieties 

 of one and the same species from each other. In the course of my 

 descriptions I shall notice several instances which bear upon this 

 point. 



That the protracted isolation of the different limestone hills had 



