224 ARTHUR WILLEY. 
The nepionic shell frequently stands in marked contrast to 
the post-nepionic or (with the large terrestrial Gastropods) 
post-natal shell, both by a definite line indicating a tem- 
porary discontinuity of growth, and by its different colour 
and minute sculpturing.! In fact, it is obvious that a 
definite break in the continuity of shell-formation is more 
likely to occur in those cases in which, after the completion of 
the nepionic shell, the young mollusc has to go through the 
operation of hatching, after which it recommences to form 
post-natal shell-substanee. This supposition is, I believe, in 
accordance with the facts. 
With regard to Nautilus, about whose reproduction we are 
so much in the dark, it is a distinct point gained to know, at 
least with respect to its shell, at what stage the animal 
hatches. 
The existence of the line of discontinuous growth, at a 
particular point, in young shells of N. pompilius, might not 
be regarded as conclusive in itself, but I have since found a 
precisely similar and constant discontinuity of growth in a 
number of shells of N. umbilicatus and N. macrom- 
phalus. 
In the two last-named species the nepionic shell differs 
from the post-nepionic shell by the presence in the former of 
a minute plication, and of a peculiar glossy sheen. In N. 
pompilius the nepionic shell also has a peculiar sheen, and 
presents a contrast to the post-nepionic shell in the colour 
of its markings, which are brick-red, those of the latter being 
crimson. 
Fig. 5 (Pl. 13) represents a drawing of the umbilical region 
of N. umbilicatus, Lam. On looking down into the um- 
bilicus at the first whorls of the shell, it will be seen that 
after the young shell had completed rather more than one 
entire whorl, there occurred a sharp and abrupt interruption 
in the growth of the whorls, and (in the actual specimen, 
although not shown in the figure) the character of the shell 
1 Compare Hepiry, Crartzs, “On the Structure and Affinities of Panda 
atomata,” Gray, ‘ Records Austral. Mus.,’ vol. ii, 1892, pls. v and vi. 
