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 highly 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 Difflugidce 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 Diffiugida 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 Dif- 

 flugidce 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 EuglyphidiB. 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, since 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 



