In a book in which an attempt is made to cover to some extent such a vast field, it would be vain to hope to have avoided many errors both of omission and commission, and I owe it to the kindness of several friends that the errors are not much more numerous. I cannot however too emphatically say that for those which remain I alone am responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham Kerr, and B. Rogers have all been kind enough to help me by reading proofs or manuscript, while the assistance that I have received from Dr Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr Shipley throughout the whole progress of the work has been very great. To all these gentlemen my best thanks are tendered. All the figures except 1, 35, 55, and 84 were drawn by Mr Edwin Wilson, to whose care and skill I am much indebted. The majority are from photographs taken by my sister Miss K.M. Reynolds or by myself in the British Museum and in the Cambridge University Museum of Zoology, and I take this opportunity of thanking Sir W.H. Flower and Mr S.F. Harmer for the facilities they have afforded and for permission to figure many objects in the museums respectively under their charge. I have also to thank (1) Prof. von Zittel for permission to reproduce figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh and Dr H. Woodward for fig. 35; (4) Dr C.H. Hurst and Messrs Smith, Elder, and Co. for fig. 55. A few references are given, but no attempt has been made to give anything like a complete list. The abbreviations of the titles of periodicals are those used in the _Zoological Record_. I have always referred freely to the textbooks treating of the subjects dealt with, and in particular I should like to mention that the section devoted to the skeleton of mammals is, as it could hardly fail to be, to a considerable extent based on Sir W.H. Flower's _Osteology of the Mammalia_. SIDNEY H. REYNOLDS. _March 10, 1897._ CONTENTS. PAGE CHAPTER I. Introductory account of the skeleton in general 1 CHAPTER II. Classification 30 CHAPTER III. Skeleton of Hemichordata, Urochordata and Cephalochordata 50 CHAPTER IV. Skeletal characters of the Vertebrata. The skeleton in the Cyclostomata 53 CHAPTER V. Skeletal characters of the Ichthyopsida. Characters of the several groups of Pisces 59 CHAPTER VI. The skeleton of the Dogfish (Scyllium canicula) 71