The eminent French conchologist Bourguignat (_a_, p. 352) was of opinion that, as far as terrestrial mollusca were concerned, there are in Europe three principal centres of creation or dispersion--all situated in mountainous countries and not in the plains. He distinguished the Spanish, Alpine, and Tauric centres, and believed that almost all species known from Europe had originated in one of these three, and that each of them possessed quite a distinct type of its own. This theory seems to agree very well with the facts of distribution. Let us take, for instance, the genus _Clausilia_, a pretty turret-shaped snail, which abounds on old ruined walls. Only two species, viz., _Cl. laminata_ and _Cl. bidentata_, are met with in Ireland. In England we find the same species with the addition of two others, _Cl. biplicata_ and _Cl. Rolphii_. Crossing over the Channel to Belgium, these four species occur again, and also several others not known in England. In Germany the list of _Clausiliæ_ mounts up to twenty-five species, including all those found in the British Islands. As we proceed eastward the number of species of this genus increases steadily, and when we reach the Caucasus or the Balkan Peninsula the conchologist is able to make a collection of several hundred different kinds, whilst farther east again they diminish. This clearly indicates there is in South-Eastern Europe a powerful centre of creation of _Clausiliæ_, from which the species have spread all over Europe. But it is by no means certain that this centre was always in our continent, for in South-Eastern Asia and the Malay Archipelago _Clausiliæ_ increase once more. It is interesting to note, however, that almost all these eastern forms belong to the sub-genus _Phædusa_ (_vide_ Boettger), which had only been known as a fossil genus from a few species in the Eocene and Oligocene of Southern Europe. The first centre of origin, therefore, may possibly have been in Southern Asia, and in these early Tertiary times a second centre may have become established in Southern Europe from which the sub-genus _Garnieria_ went eastward, _Macroptychia_ southward, and _Nenia_ westward across the Atlantic to South America. Only a few remnants of these primitive _Clausiliæ_ are now left in Europe, such as the interesting _Cl. (Laminifera) Pauli_.