BHIM TAL[H] is a lake situated at an altitude of 4500 feet in that part of the Western Himalayas known as Kumaon, near the plains. It has a superficial area of several square miles, and is deep in the middle. Its bottom and banks are for the most part muddy. Little is known of its fauna, but two polyzoa (_Plumatella allmani_ and _Lophopodella carteri_) and the gemmules of two sponges (_Spongilla carteri_ and _Ephydatia meyeni_) have been found in it. [Footnote H: The fauna of this lake and of others in the neighbourhood has recently been investigated by Mr. S. W. Kemp. See the addenda at the end of this volume.--_June 1911._] * * * * * NOMENCLATURE AND TERMINOLOGY. The subject of nomenclature may be considered under four heads:--(I.) the general terminology of the various kinds of groups of individuals into which organisms must be divided; (II.) the general nomenclature of specimens belonging to particular categories, such as types, co-types, etc.; (III.) the nomenclature that depends on such questions as that of "priority"; and (IV.) the special terminology peculiar to the different groups. The special terminology peculiar to the different groups is dealt with in the separate introductions to each of the three parts of this volume. (I.) No group of animals offers greater difficulty than the sponges, hydroids, and polyzoa (and especially the freshwater representatives of these three groups) as regards the question "What is a species?" and the kindred questions, "What is a subspecies?" "What is a variety?" and "What is a phase?" Genera can often be left to look after themselves, but the specific and kindred questions are answered in so many different ways, if they are even considered, by different systematists, especially as regards the groups described in this volume, that I feel it necessary to state concisely my own answers to these questions, not for the guidance of other zoologists but merely to render intelligible the system of classification here adopted. The following definitions should therefore be considered in estimating the value of "species," etc., referred to in the following pages. _Species._--A group of individuals differing in constant characters of a definite nature and of systematic[I] importance from all others in the same genus. [Footnote I: "What characters are of systematic importance?" is a question to which different answers must be given in the case of different groups.] _Subspecies._--An isolated or local race, the individuals of which differ from others included in the same species in characters that are constant but either somewhat indefinite or else of little systematic importance. _Variety._--A group of individuals not isolated geographically from others of the same species but nevertheless exhibiting slight, not altogether constant, or indefinite differences from the typical form of the species (_i. e._, the form first described).