No. 5. — Fossil Cephalopods of the Museum of Comparative Zoology. Embryology, by Alpheus Hyatt. The researches recorded in the following pages were originally un-dertaken in order to ascertain the limits of the embryological period among the typical Ammonites. During this period, which begins with the ovisac, the different species possess a common form, and are very similar in the characteristics of their septa, siphons and shells. It was at first proposed to give the descriptions only in conjunction with the different groups to which the young belonged ; but the intimate connection and importance of the facts, elicited by a general comparison of the young of Nautilus, Goni-atites, and Ammonites appeared to demand a separate publication. The necessary illustrations have been furnished with unstinted liber-ality by the Director of the Museum, Professor Louis Agassiz. The collections also, which are extremely rich, have been placed entirely at my disposal, and I have been permitted to break up whatever speci-mens were considered suitable for the present purpose. As this is my second formal publication upon the Cephalopods in the Bulletin, it is only becoming to correct certain errors which are to be found in the first (No. 5, Vol. I, Bulletin of the Museum of Comp. Zoology), under the same general title as this number. Proper credit was not given in the preface to Professor Edward Suess for having been the first to publish the fact, that the typical forms of Ammonites were capable of generic division, and two of his names, Lytoceras and Phylloceras, should supersede two of those given in that number of the Bulletin, namely Thysanoceras and Rhacoceras. I have been rather severely criticised by Laube and Zittel for giving Professor Agassiz the credit of having been the first to perceive that the Ammonites were divisible into distinct families and genera, but it will be noticed that this is given to him as a personal matter between an instructor and his student. This I must be excused from withdrawing. But I did know, however, that ever since Professor Agassiz published his French translation of Sowerby's Mineral Conchology, he has re-garded the Ammonites, not as a family, as Suess does, but as a large group, perhaps equivalent to a sub-order, and composed not of a few genera, but of several families containing many genera.