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. 



