In discussing the aquatic fauna of Europe we are accustomed to divide it into two sections, the marine fauna and that of fresh water. With a few exceptions, such as that of the Decapod Crustacean Palaemonetes varians in brackish water in Northern Europe (and in fresh water in the Mediterranean region) and of certain molluscs found in the estuaries of all our large rivers, the separation between these two elements is fairly constant, and when marine species such as Mysis relicta are found in inland lakes the facts are considered exceptional. In tropical waters, however, conditions are different and we find not only a fairly large fauna of brackish water which cannot be regarded either as strictly marine or as strictly fluviatile, but also a considerable number of marine types that have established themselves permanently in fresh water far above the influence of the tides. Among the problems that confront the biologist and the student of zoogeography few have greater interest than that of the origin of freshwater faunas, or rather of the fresh water fauna. For over the greater part of the world the animals that inhabit rivers and lakes have a great similarity and, indeed, include a large element of cosmopolitan genera and even species. This cosmopolitan element is of ancient origin. In India it can be traced back to the late Cretaceous beds of the Inter-Trappean period, in which species of such cosmopolitan molluscan genera as Limnaea, Vivipara and Corbicula occur in abundance. In other countries its known range in time is very much greater ²), and the fact that even in the Inter-Trappean beds certain species already exist that are very closely allied to living forms from the swamps and pools of the same districts, proves that when these beds were deposited the genera had already been long established and had attained a high degree of stability I may cite as examples the Inter-Trappean Limnaea oviformis ¹), Vivipara normalis ²) and Lamellidens vredenburgi ³), and their modern representatives 4) Limnaea luteola Lamarck, Vivipara dissimilis (Müller) and Lamellidens marginalis (Lamarck). In each pair of species the resemblance is so close, so far as can be judged from the shells alone, that the living mollusc appears to be hardly more than a variety or race of its deceased ancestor. And yet the Inter-Trappean forms were contemporaneous with Dinosaurs 5) and possibly even entered into their diet.