REMARKS ON CONCIIOLOGY AND MALACHOLOGY. 231 
some kinds are very liable to it, particularly the genus Nassa, ma¬ 
ny species of which are common here. In illustration of this 
property of change, I shall describe a species of lS T assa found in 
the mud of salt swamps: it is in colour a dark brown or black, 
about an inch and a half in length, the outer whorl is smooth, 
those next the apex of the spire are furrowed longitudinally, and 
it possesses the usual generic mark of a prominent plait at the 
upper part of the aperture. Out of many specimens examined I 
have observed none to deviate from the above description. In the 
same localities may be found another shell quite similar to the 
other in form and colour, but not more than half its length, 
possessing however all the marks of a full grown shell, and as no 
shells of intermediate size are to be met with, there seems good 
reason to believe them two distinct species. The following instance 
is however more remarkable in connexion with the above. I late¬ 
ly found at Malacca a small species of Nassa of a pale flesh co¬ 
lour, barred with brown, about a third of an inch in length, and 
little more than a grain in weight. In the same neighbourhood 
I met with another specimen, three quarters of an inch in length, 
and weighing between four and five grains. As in the former 
case, the two shells were exactly similar in shape and colour, 
though very different in size and. weight, and as both had the 
marks of having attained their full size, I was ready to believe 
that I had obtained two new species; a further search however, 
put me in possession of fifteen additional specimens, similarly mark¬ 
ed but all of them intermediate to the two first in size and weight ; 
in fact the whole seventeen formed an almost imperceptible scale 
of gradation, sufficiently proving that they w ere so many varieties of 
one and the same species. I have observed several kinds of 
Isassa particularly abundant in the neighbourhood of the Fish Mar¬ 
kets, where they may be seen in numbers feeding on dead fish 
and other animal food. This artificial mode of subsistence is possibly 
one cause of their variable form and size as it is well known 
that domestie animals, and others that are more or less depen¬ 
dent on man for their support, are very apt to produce a pro¬ 
geny differing more or less from the parent stock, A good 
example of the propogation of an accidental variety, must be 
familiar to my readers in the instance of a well known domestic 
