266 The Journal of the Indian Archipelago. 



change of food, &c, that uniformity of colour, size or even shape, 

 when taken separately, are no safe guides, and unfortunately they 

 are not always found combined. Blumenbach wisely observes that, 

 * no general rule can be laid down for determining the distinctness 

 of species, as there is no particular class of characters which can 

 serve as a criterion.' 



" This variable tendency does not prevail in all shells, though 

 some kinds are very liable to it, particularly the genus Nassa, many 

 species of which are common here. In illustration of this property 

 of change, I shall describe a species of Nassa 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 lately found at Malacca a small species of Nassa 

 of a pale flesh colour, barred with brown, about a third of an inch in 

 length, and little more than a grain in weight. In the same neigh- 

 bourhood 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 marked, 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 were so many varieties of one and the 

 same species. I have observed several kinds of Nassa particularly 

 abundant in the neighbourhood of the fish markets, where they 

 may be seen in numbers feeding on dead fish and other animal food. 



