Hyatt.] . 396 [December 20, 



Arthur N. Austin, and upon his motion a vote of thanks for 

 the gift was passed. 



Prof. Hyatt spoke of the results of his researches on the 

 embyology and development of the Shells of the Ammonoids 

 and Nautiloids, made at the Museum of Comparative Zoology 

 in Cambridge. 



The ovisac of Ammonites was discovered by Louis Sacmann and 

 figured in Dunkee and Meyer's Paleontographica. A figure of Nau- 

 tilus aratus was also drawn alongside of this in order to show the con- 

 trast of its obtusely pointed apex with the globular apex of the first 

 whorls of Lytoceras jimbriatus. 



Other authors before this had regarded the Goniatites as the only 

 forms of tetrabranchiate Cephalopods which possess globular ovi- 

 sacs, and had characterized the young in this way. I have been able 

 to verify Sacmann's observations with his original specimens, so that 

 we must now consider all the Ammonoids as beginning with a globu- 

 lar ovishell. 



Nautilus, also, though it never retains an ovisac, shows at the apex 

 a scar which marks the former attachment of some such embryonic 

 shell. This scar has such a form that one is led immediately to com- 

 pare it with the outline of the aperture and living chamber of Gom- 

 phoceras. 



The aperture of the embryo and the form of the living chamber, as 

 indicated by the scar, has its longest axis vertical or abdomino-dorsal 

 instead of from side to side, as in many Goniatites and all of the typi- 

 cal Ammonites. 



No signs of this scar were directly observed among fossil Nautiloids, 

 though indications of its presence were not wholly wanting, especially 

 among Jurassic Nautili. 



The variability in the form of the young and its amount of coiling 

 is much greater in the earlier than in the later formations. Among 

 Silurian species the young may be straight like Endoceras, and as the 

 siphon is probably central at this age and composed as in Endo- 

 ceras of a succession of siphonal funnels, we must have with this 

 agreement of form an agreement of structure also. These straight 

 young form the nuclei of varieties with elliptical adults, and the pas- 

 sage is insensible from these to others of the same species having 

 young which are closely coiled from the very beginning. 



