Transcriber Note Text Emphasis denoted as _Italics_ and =Bold=. Magnificatons as 30/1. The » is used in Swedish for quotes and dittos. Numeric abbreviations N:o, 1:o, 2:o, etc. for Number, primo, secundo, etc. Commas are used here as decimal separators (ex., 0,06 = 0.06. KONGL. SVENSKA VETENSKAPS-AKADEMIENS HANDLINGAR. Bandet 34 N:o 8. RESEARCHES ON THE VISUAL ORGANS OF THE TRILOBITES BY G. LINDSTRÖM. WITH SIX PLATS COMMUNICATED TO THE ROYAL SWEDISH ACADEMY OF SCIENCES SEPTEMBER 12TH 1900. STOCKHOLM KUNGL. BOKTRYCKERIET. P. A. NORSTEDT & SÖNER 1901. CONTENTS. Introduction Page 6. Blind Trilobites » 9. The eyes of the trilobites » 26. On the maculæ of the hypostoma » 35. Conclusions » 71. Explanation of the plates » 75. At the outset of this memoir it must be said, that it is in fact a joint work by the first discoverer of the remarkable hypostomic structure to be described, Herr G. LILJEVALL, and by myself. A considerable portion of this work belongs to him as he with great discernment searched for a large amount of the material, prepared it as to be fit for the many microscopic sections which he executed, and at last with his well known artistic skill drew the figures. My share, again, in the common work consists in the supervision of the figures, in the arrangement of the observations and details in a coherent form, in which suggestions and remarks of Herr LILJEVALL have been of great use, further to write the descriptions and to make all the necessary researches in literature, so whatever may be faulty in these respects must be laid to my charge. September 12th 1900. G. LINDSTRÖM. INTRODUCTION. It is well known amongst palæontologists that the hypostoma of the trilobites has been the particular object for research and description by a few authors, as BARRANDE, NOVÁK and BRÖGGER and some passing notes concerning it are given in the descriptive works of other authors. In the following pages reference is often made to them and I may here give a list of the chief works and the abbreviations with which they are cited below in this paper.