The first volume contains, in chronological order, all Balfour's scattered original papers, including those published by him in conjunction with his pupils, as well as the Monograph on the Elasmobranch Fishes. The last memoir in the volume, that on the Anatomy and Development of Peripatus Capensis, was published after his death, from his notes and drawings, with additions by Prof. Moseley and Mr Adam Sedgwick, who prepared the manuscript for publication. To the volume is prefixed an introductory biographical notice. The second and third volumes are the two volumes of the Comparative Embryology reprinted from the original edition without alteration, save the correction of obvious misprints and omissions. The fourth volume contains the plates illustrating the memoirs contained in Vol. 1. We believe that we are consulting the convenience of readers in adopting this plan, rather than in distributing the plates among the memoirs to which they belong. To assist the reader the explanations of these plates have been given twice: at the end of the memoir to which they belong (in the case of the Monograph on Elasmobranch Fishes at the end of each separate chapter), and in the volume of plates. All the figures of these plates had to be redrawn on the stone, and our best thanks are due to the Cambridge Scientific Instrument Company for the pains which they have taken in executing this work. We are also indebted to the Committee of Publication of the Zoological Society for the gift of electrotypes of the woodcuts illustrating memoir no. XX. of Vol. 1. Several photographs of Balfour, taken at different times of his life, the last shortly before his death, are in the possession of his relatives and friends; but these, in the opinion of many, leave much to be desired. There is also a portrait of him in oils painted since his death by Mr John Collier, A.R.A., and Herr Hildebrand of Florence has executed a posthumous bust in bronze[1]. The portrait which forms the frontispiece of Vol. 1. has been drawn on stone by Mr E. Wilson of the Cambridge Scientific Instrument Company, after the latest photograph. Should it fail, in the eyes of those who knew Balfour well, to have reproduced with complete success his features and expression, we would venture to ask them to bear in mind the acknowledged difficulties of posthumous portraiture. Footnote 1: In possession of the family. Copies also exist in the Library of Trinity College, and in the Morphological Laboratory, at Cambridge. TABLE OF CONTENTS. PAGE PREFACE i INTRODUCTION 1 1872 I. On some points in the Geology of the East Lothian Coast. By G. W. and F. M. BALFOUR 25 1873