ZOOLOGICAL OBSERVATIONS IN THE SOUTH PACIFIC. 227 
IDOE 
On some VARIATIONS IN THE SHELL oF NAUTILUS, WITH 
Description or A New Variety (N. POMPILIUS, VAR. 
MoRreETONI, NOV. VAR.). 
(With figs. 7, 8, and 9 on Plate 13.) 
Of the obvious differences that exist between the shell of 
N. umbilicatus and that of N. pompilius the following 
may, for my present purpose, be particularly remembered. 
1. In N. umbilicatus (see Pl. 13, fig. 5)— 
a. The umbilicus is perforated, the shell being so loosely 
coiled that a small aperture, which can be looked through, is 
left in the centre of the whorls. 
(3. The umbilicus is large and open, so that all the whorls 
can be distinctly seen. This holds good also for N. macrom- 
phalus, but here the umbilicus is not perforated. 
y. The free margin of the shell does not merge into the 
umbilicus, but tends abruptly inwards and downwards, so as to 
form a prominent shoulder, nearly at right angles to the 
lateral free margin. It finally fuses laterally with the anterior 
convexity of the shell, which is pigmented black. 
2. In N. pompilius— : 
a. The umbilicus is imperforate (excepting very young 
individuals). 
. [3. The umbilicus is practically obliterated through a depo- 
sition of callus by the dorso-lateral angles of the mantle. 
y. The free margin of the shell merges directly into the 
umbilical region. 
The different characters tabulated above are usually very 
distinct, but they may be considerably weakened by variation. 
One variety, for instance, of N. pompilius will present all 
the characteristics of the species with the single exception 
that the free margin of the shell does not pass directly into 
the umbilicus, but forms a shoulder. 
In another variety the free margin of the shell will pass 
gradually into the umbilicus, but the latter will be perforated. 
