BY WILLIAM A. HASWELL, 
545 
inches; this part was laterally compressed; at its basis its measure 
from the dorsal to the ventral side was found to be 1 inch, 10 
lines; from the right to the left side only 1 inch." He adds "This 
part was proved to me by dissecting it to be formed by the union 
of four unusually developed tentacular slips, one of which was 
shorter and more free, the three other chiefly composing the sin- 
gular body." 
From the dimensions here given and the figures accompanying 
the paper it would appear that Van der Hoeven had under obser- 
vation a completely or nearly completely developed specimen. Such 
mature specimens are comparatively rare; and all the male 
Nautili that have been made the subjects of other published 
descriptions appear to have been immature, so that some important 
and interesting points in the structure of the fulty-developed 
spadix have been overlooked. 
In the larger of the two mature male specimens I have 
had the opportunity of examining (in which the greatest 
diameter of the shell is 6J inches) the total length of the 
organ is 3 J inches, the greatest breadth about inch, and 
the thickness a little less than an inch. The four tentacles com- 
posing it (Plate xlviii. fig. 1) are all very strongly modified in 
different directions. One of them, as observed by Van der 
Hoeven, is separate from the rest except at the base. It is shorter 
than the others, and does not seem to be capable of being 
retracted, its sheath being very short: its free part, which is 
spathulate towards the extremity, lies under shelter of a wide fold 
extending backwards over it from the sheath of the tentacle 
which I have numbered 3. Tentacles 2, 3, and 4 have their 
sheaths united, but the tentacles themselves are quite separate. 
No. 2 is a thick, solid, muscular cylinder, or rather elongated, 
blunt cone, probably not capable of being extended to any great 
distance; the cavity of its sheath is very wide. No. 3 is elon- 
gated and laterally compressed, marked on its posterior and external 
surface with numerous transverse ridges. No. 4 presents the 
most remarkable modification; it is thick and cylindrical towards 
the base, becoming compressed towards the free end. The outer 
