490 Miscellaneous. 
and silvery white on the ventral surface. Towards the end of 
March it had lost its dark colour and was of a palish olive-yellow, 
but the red colour, which seems to be connected with the phenomena 
of reproduction, and which is frequently very vivid in full-grown 
males about the region of the head and gills, was slightly developed. 
On the 30th March my fish made a curious small excavation in the 
sandy bottom of the aquarium, and for the next two days I observed 
that it went frequently to this nest, apparently for the purpose of 
sexual deposition. On the 1st April I obtained a further supply of 
sticklebacks, which were placed in the aquarium on the evening of 
the same day. These were dark in colour, and presented a marked 
contrast to the pale yellow of the original tenant. It is needless 
to say that violent warfare ensued upon the introduction of the 
strangers. The nest was destroyed almost immediately, and its 
erstwhile proprietor devoted itself to making savage attacks upon 
the newcomers. On examining the pale-coloured individual about 
half an hour later it appeared to me, and to a friend who. had 
throughout followed my observations, that it had assumed a some- 
what darker hue, while in two hours it could not be distinguished 
from the darkest of the later introductions. We had, however, no 
difficulty in still recognizing the original specimen by certain habits 
it had acquired during its solitary confinement, and notably by the 
fact that it would attack the end of the finger or a pencil placed in 
the water, whereas the new arrivals immediately retreated. 
This rapid and complete change of colour struck me as very 
remarkable, the more so as it seems quite impossible to account for 
it. No change was made in the quality of the water, nor was there 
any change in the quality or quantity of light, even for the shortest 
time, so that the alteration observed seems to be attributable solely 
to inexplicable sympathetic influence. I am not aware that such a 
change has heretofore been observed in any animal, and I thought it 
might be worth recording in your pages. 
On Sexual Dimorphism in the Nautilus. By A. Vayssikrn. 
Our knowledge of the external sexual differences of Cephalopods 
is in many respects still deficient. 
As Lhad the privilege, in 1887, of examining some specimens of 
Nautilus macromphalus, Sow., and this year of studying a certain 
number of VV. pompilius, L., I have thought that it would be inter- 
esting to point out the external sexual dimorphism that is found to 
exist in these mollusks. 
On examining a certain number of Nautilus shells, taking care to 
place them with their mouths facing us and the outer lips of the 
latter uppermost, we find that the shape of the mouth of the shell 
is somewhat variable. The orifice is sometimes ovoido-conical, 
