226 ARTHUR WILLEY. 
There is strong presumptive evidence that the animal of 
Nautilus at the time of hatching already possesses the main 
features of the adult, with the possible addition of a yolk-sac, 
This follows both from the size of the nepionic shell, which 
comprises a number of chambers, and from the consideration 
of a very small specimen of Nautilus, which I obtained in 
New Britain and mentioned in a former publication.!, I have 
unfortunately mislaid the shell of this specimen, but it had a 
perforated umbilicus, through which daylight could be seen, 
and was at least as small as the shell from which Pl. 13, fig. 6, 
was drawn. 
Jackson (loc. cit.) refers to Hyatt’s figures of a young 
Nautilus Koninckii, in which is indicated “a smooth ne- 
pionic period, succeeded by a fluted neologic stage.” Ihave . 
been unfortunately unable to refer to Hyatt’s well-known 
memoir on the “ Embryology of Fossil Cephalopoda ”’ (‘ Bulletin 
Mus. Compar. Zool. Cambridge, Mass.,’ vol. iii, 1872). 
In the first part of his “ Beitrage zur Entwickelungsge- 
schichte der fossilen Cephalopoden” (‘ Paleontographica,’ 
vol. xxvi, 1879-80) Branco figures a number of minute shells 
of Ammonites, which sometimes, when they measure even less 
than 1 mm. in diameter, exhibit a deep circular constriction ; 
and this not only in cases in which the adult shell is grooved, 
but also in many instances in which the latter is not grooved. 
If this groove were to be identified or compared with the 
nepionic line of recent Nautilus, which is itself sometimes 
depressed, as mentioned above, it would indicate that the ova 
of the Ammonites referred to were much smaller and poorer in 
yolk than those of tne recent Nautilus. This would be an 
interesting conclusion if it could be substantiated. 
1 * Natural Science,’ June, 1895. 
