DR. COOKE is justified in saying, in his preface, that no apology is needed for the production of this volume, the latest addition to the “International Scientific Series.” Notwithstanding the increased attention which has been paid of recent years in this country to this interesting and beautiful class of plants, we have had hitherto no popular hand-book devoted to their structure and their classification; the only existing works on British freshwater Algæ, by Hassall, and by Dr. Cooke himself, having been published at a price which places them out of the reach of the great majority of collectors of Algæ. Introduction to Freshwater Algæ, with an Enumeration of all the British Species. By M. C. Cooke With 13 Plates. (London: Kegan Paul, Trench, Trübner, and Co., Limited, 1890.)