230 Dr, G,C, Wallich on the Extent and Causes of 
genera) as a chitinoid material. By this expression it is simply 
intended to convey that the material referred to presents as close 
an approach to the substance known as chitine as is determinable 
from the optical characters of portions far too minute to admit 
of chemical analysis ; and I believe I merely adopt a view very 
generally entertained by competent observers, when I give it this 
designation. But whatever may be the precise chemical composi- 
tion of the substance, the name serves to indicate a well-known 
part of the structures ; and, to say the least of it, this is a higher 
quality than can be assigned to many of the scientific terms 
which are now and then submitted to the public. 
In the majority of the Difflugian tests, this chitinoid material 
forms a continuous and smooth layer internally, it being on the 
external surface alone that mineral particles are impacted. It 
is also a most interesting fact, that no vegetable or extrinsically 
derived animal substances are employed for the consolidation of 
the test, and that the particles selected are, I believe invariably, 
of mineral nature. On the other hand, it is manifest that the 
selective power is carried to such an extent that colourless par- 
ticles, sometimes quartzose, sometimes felspathic, sometimes 
micaceous, are always chosen* —the absence or presence of 
angularity in these particles being of course dependent on the 
condition of the sandy matter in each locality. 
The particles would seem to be impacted into the chitinoid 
matrix just in the same way that a brick is pressed imto the 
yielding mortar; and this too in so skilful a manner as to leave 
the smallest possible amount of vacant area; whilst in the 
specimens in which tabular or micaceous particles are used, these 
are sometimes disposed with such nicety that there is no over- 
lapping, but the small fragments are placed so as to occupy the 
spaces left between the larger ones, Figs. 11, 15,20, in Plate 
XVI., are examples of this kind +. 
It is curious that even in pools or streamlets in which the 
deposits seem to consist almost exclusively of vegetable débris, 
the Difflugie still manage to find mineral matter sufficient for 
their purposes; whilst, as already stated, in those places where 
they run a risk of being washed away by running water, they 
reduce the chances of the catastrophe as far as possible by 
loading their tests with the largest particles and the greatest 
quantity of mineral matter. Figs. 9 & 10, the one from Green- 
land, the other from a little streamlet at Hampstead, are mode- 
rate examples of this loading, which, it may be remarked, is 
* Tn Indian specimens I have occasionally detected the siliceous spicules 
of Spongilla. 
+ Plate XVI. will now alone be referred to, unless the contrary be 
expressly stated. 
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