Classes. I. Cephalopoda Cuttle-fish. II. Gasteropoda Snails and aquatic Univalves. III. Pteropoda Oceanic Snails. IV. Brachiopoda Symmetrical Bivalves. V. Conchifera Unsymmetrical Bivalves. The Gasteropoda and Conchifera alone contain land and freshwater forms, and to these we shall chiefly confine our illustrations of the geographical distribution of the Mollusca. The classification followed is that of Dr. Pfeiffer for the Operculata and Dr. Von Martens for the Helicidæ. The families chiefly referred to are:-- Class II.--GASTEROPODA. Order 2.--Pulmonifera. Fam. { 1. Helicidæ. { 2. Limacidæ. { 3. Oncidiadæ. In-operculata { 4. Limnæidæ. { 5. Auriculidæ. { 6. Aciculidæ. { 7. Diplommatinidæ. Operculata { 8. Cyclostomidæ. { 9. Helicinidæ. PART II. _ON THE DISTRIBUTION OF EXTINCT ANIMALS._ {107}CHAPTER VI. THE EXTINCT MAMMALIA OF THE OLD WORLD. Although it may seem somewhat out of place to begin the systematic treatment of our subject with extinct rather than with living animals, it is necessary to do so in order that we may see the meaning and trace the causes of the existing distribution of animal forms. It is true, that the animals found fossil in a country are very generally allied to those which still inhabit it; but this is by no means universally the case. If it were, the attempt to elucidate our subject by Palæontology would be hopeless, since the past would show us the same puzzling diversities of faunas and floras that now exist. We find however very numerous exceptions to this rule, and it is these exceptions which tell us of the past migrations of whole groups of animals. We are thus enabled to determine what portion of the existing races of animals in a country are descendants of its ancient fauna, and which are comparatively modern immigrants; and combining these movements of the forms of life with known or probable changes in the distribution of land and sea, we shall sometimes be able to trace approximately the long series of changes which have resulted in the actual state of things. To gain this knowledge is our object in studying the "Geographical Distribution of Animals," and our plan of study must be determined, mainly, by the facilities it affords us for attaining this object. In discussing the countless details of distribution we shall meet with in our survey of the zoological regions, we shall often find it useful to refer to the evidence we possess of the range of the group in question in {108}past times; and when we attempt to generalise the phenomena on a large scale, with the details fresh in our memory, we shall find a reference to the extinct faunas of various epochs to be absolutely necessary.