Mr. Lydekker undertook the task of arranging the various articles in their proper sequence, selecting from these such portions as seemed suitable, filling up the gaps, and adding new matter where necessary; a large amount of this new matter treating of the extinct forms, and also of the group Artiodactyla. The subsequent revision, both before being sent to the printers, and also when passing through the press, has been made by both authors, who are thus jointly responsible for the whole work. The illustrations are to a great extent those prepared for the various articles in the Encyclopædia, but many have been added—some drawn expressly for the work, and some borrowed from other publications. For most of the latter the authors take this opportunity of expressing their thanks to the Publication Committee of the Zoological Society of London, as well as to the individual writers in whose works they first appeared. The authors have further much pleasure in acknowledging the ready and obliging way in which Mr. Oldfield Thomas has, throughout the progress of the work, placed his extensive knowledge of the group of animals of which it treats at their disposal. LONDON, _March_ 1891. CORRIGENDA. Page 280, _for_ Chæropsis _read_ Chœropsis. Page 292, _for_ Chæropotamidæ and Chæropotamus _read_ Chœropotamidæ and Chœropotamus. Page 590, _for_ Pæcilogale _read_ Pœcilogale. =Transcriber’s Note:= The corrections have been applied. CONTENTS PAGE CHAPTER I INTRODUCTORY REMARKS 1 Use of term mammals, 1; Characters of mammals, 2; Development of young, 3; Size of mammals, 4; Uses and products of mammals, 4. CHAPTER II GENERAL ANATOMICAL CHARACTERS 7 I. Tegumentary Structures 7 Hair, 7; Colour, 8; Scales, etc., 11; Nails, claws, and hoofs, 12; Odour-secreting glands, 12. II. Dental System 13 Teeth, 13; Structure of teeth, 13; Development of teeth, 15; Forms of teeth, 17; Succession of teeth, 19; Arrangement, homologies, and notation of teeth, 21; Dental formulæ, 25; Modifications of teeth in relation to function, 28; Taxonomy, 30; Trituberculism, 30. III. The Skeleton 33