330 . ■ 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 DifHugian 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 into 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 f. 



It is curious that even in pools or streamlets in which the 

 deposits seem to consist almost exclusively of vegetable debris, 

 the Difflugice 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 



* In Indian specimens I have occasionally detected the siliceous spicules 

 of Spongilla. 



t Plate XVI. will now alone be referred to, unless the contrary be 

 expressly stated. 



