ZOOLOGICAL OBSERVATIONS IN THE SOUTH PACIFIC. 225 
added after this line,! is very distinct from the previously 
formed shell, as already mentioned. Sometimes there is a 
visible depression in the shell at the nepionic line. 
In the shells of all species of Nautilus there are sometimes 
to be seen one or several strongly marked lines indicating 
occasional, but not regularly periodical, interruptions in the 
growth of the shell,—periods of rest, in fact. These, however, 
are neither constant in position nor in their occurrence in 
different individuals. On the other hand, the nepionic line 
occurs in a constant position in all individuals of a species, 
although in N. pompilius it cannot be seen beyond a certain 
stage, owing to the overgrowth of the whorls. 
Altogether I have seen the nepionic line in eighteen shells 
of the three above-named species of Nautilus. 
Fig. 6 (Pl. 18) is an outline sketch of a young shell of 
N. pompilius? with still perforated umbilicus, to show the 
nepionic line marking off the prenatal from the post-natal 
shell. ‘The following are the measurements of this shell: 
(a) Length of complete shell (from the anterior free margin 
to a point directly opposed to it on the posterior convex surface 
of the shell), 32 mm.; greatest width of complete shell, 19 mm. 
(b) Length of nepionic portion of shell, 27 mm. (this varies 
slightly in different individuals from 25 to 27 mm.); greatest 
width of nepionic portion of shell, 16 mm. 
It may be observed that the above dimensions of the nepionic 
shell of N. pompilius are a good deal in excess of those of 
the ovarian ovum which I previously described ; but not more 
so relatively, I think, than is the size of other newly hatched 
Cephalopods to the original size of the freshly deposited ova or 
of the mature ovarian ova. 
Knowing the size and character of the nepionic shell of 
recent Nautilus, and the size and character of its submature 
ovarian ovum, I think there is ample justification for the con- 
clusion that the nepionic line marks the period at which the 
young individual hatches from the egg. 
1 T shall refer to this in future as the “ nepionic line.” 
* The sketch was taken from a specimen in the Australian Museum by 
kind permission of Mr. Charles Hedley. 
VOL. 39, PART 2.—NEW SER. P 
