Structural Variation among the Difflugian Rhizopods. 231 
more frequent in the mitriform than in the globular series. 
It also takes place more frequently in certain varieties than in 
others of the globular series, as will be seen on reference to figs. 
17, 18, & 27 on the one hand, and figs. 19, 20, 22, & 23 on 
the other. 
But the selection of mineral particles is not confined to strictly 
inorganic substances. The Difflugie seem to know that in 
the valves of the Diatomacee are combined the properties best 
suited to their wants—that is to say, transparency and forms capa- 
ble of being easily arranged,—at the same time that the diatoms 
occur as epiphytes on the aquatic plants upon which they them- 
selves frequently find feeding-ground. And itis a remarkable cir- 
cumstance that we can generally tell whether diatoms are or are 
not plentiful in a given locality by observing the share taken by 
them in the composition of the Difflugian tests. Thus, to cite an 
example close at hand, in the Hampstead pools the predominant 
diatoms are Pinnularia and Eunotia—the former of very large 
size, the latter extremely minute *. Hence the first is but rarely 
seen impacted into the tests, and when present it is of medium 
size. The second, however, constantly occurs, and, in the curious 
variety of test referred to as D. spiralis, and likewise in the glo- 
bular and lageniform series, they often constitute a very large 
percentage of the mineral matter (figs. 18, 18a, 244, & 32+). 
In the Greenland mountain streamlets from which I obtained 
some of my specimens, Eunotias occur very abundantly, and under 
a remarkable variety of forms. In some tests they constitute the 
entire mass of mineral matter, and, as in the Hampstead ma- 
terial, pervade nearly every shape of test, though much less pre- 
dominant in the depressed series (figs. 9 & 27). 
_ [ have now to speak of several novel modifications in the out- 
ward characters of the Difflugian tests which have heretofore been 
observed by me, for the most part amongst the varieties of the 
mitriform and lageniform series, and which I have been able, only 
within the past few months, to trace as an unbroken chain from 
the forms already described to those in which the chitinoid matrix 
presents no appreciable admixture with mineral matter. 
The first indication of this very remarkable and instructive 
* This diatom would seem to have been hitherto undescribed. It is 
stipitate, the valves being generally from 35155 to 7,45 of an inch in length, 
and occurs in crowds around a filamentous “ frond,” appearing to be very 
generally distributed. An extremely minute Navicula also occurs, but is 
rarely employed in the construction of the test—thus affording another 
example of selective power. 
+ The two figures 24 & 24a are not intended to show these diatom- 
valves; but certain cylindrical or, as I formerly called them (‘ Annals,’ 
June, p. 451), pellet-shaped cylinders, which will be more particularly re- 
ferred to presently. 
