238 Dr. G. C. Wallich on the Extent and Causes of 
Rhizopods. Indeed, in point of number and variety, the latter 
were quite as abundant in Greenland as in Bengal; whilst the 
dimensions of the Greenland specimens were only inferior to 
those of some of the most lighly developed varieties from the 
Gangetic Sunderbunds, to which I have already drawn attention. 
I have now to speak of the last and certainly the least im- 
portant of the modifications, namely, colour. My remarks on 
this head shall, therefore, be very brief. 
Where the water in which the Difflugide occur is pure, and 
the vegetable matter contained in it not undergoing decay, the 
tests of these organisms, as might be inferred from the nature of 
the materials of which they are composed, are colourless. Some- 
times mud or disintegrating organic matter adheres to the 
tests, and imparts a tint ; but it must be obvious that this bears 
no relation to the animal or even to its test, and is therefore 
as variable a character as it is accidental. The chitinoid sub- 
stance, however, which constitutes the basis of the matrix of the 
tests, generally speaking, exhibits a delicate sienna-tint; but 
sometimes it assumes a darker shade, and hence imparts 
a similar colour to the entire test. But, as this peculiarity 
pervades the entire series of forms, it furnishes no distinctive 
character. In some cases, as, for example, where Difflugide are 
collected in the red muddy deposits met with at Hampstead and 
elsewhere, the tint above referred to may be imparted from 
without; and since we know that in D. Arcella the young 
tests are colourless, whereas the mature ones gradually become 
brown, it is reasonable to suppose that the effect depends on 
age or exposure. But, for the reasons assigned, whilst the 
deepening of the colour of the test in D. Arcella enables us to 
form some estimate of the age of a specimen, in the other Dzf- 
Jlugide the effect takes place to such a limited extent as to be 
unavailable for this purpose. It only remains to be mentioned 
that the colour of the animal within the tests varies, from the 
pale gray of granular sarcode to green or yellow or brownish red ; 
but, in my experience, I have invariably been able to trace all — 
the last-mentioned modifications of tint to the nature of the food 
which the organism has incepted. 
My present limits do not admit of more than a cursory allu- 
sion to the Euglyphide. Indeed it would be impossible to give 
a satisfactory outline of their characters and relations without — 
introducing several new and undescribed marine forms which © 
would obviously be out of place in the present memoir. I shall 
confine myself, therefore, to stating that, sce no doubts have 
arisen concerning the identity of the animal in the several genera 
and species into which the heretofore described members of this 
group have been subdivided, and mere differences in the figure 
