224. Dr. G. C. Wallich on the Extent and Causes of 
test is so far asymmetrical (and in this respect differs in cha- 
racter from all those previously described, with the exception of 
D. spiralis), that a section passing transversely through the 
apertural plane would form two very unequal portions. Never- 
theless it can be shown that even here there is no valid ground 
for assuming specific distinctness, or that the figure of the test 
is not determined by eatrinsic conditions. 
I have already explained how readily the slightest inequality in 
the distribution of the mineral particles in the earliest state of the 
test may cause it to assume an oblique figure, and that such figure 
may, without any improbability, be also imparted to it, in the 
young state and (as will hereafter be seen) at subsequent periods of 
its history, by the action of running water,—D. spiralis affording 
a marked example, in which the latter agency would seem to pro- 
duce the effect in the most signal degree*. Now, although I 
have heretofore been unable to satisfy myself that in habitats 
in which there is a current, and in those in which there is none, 
the asymmetrically and symmetrically shaped tests respectively 
predominate to such an extent as to leave no room for doubt as 
to the efficacy of the second cause I have suggested, from such 
observations as I have made on the character of the spots from 
which I have obtained Difflugia, I cannot help believing that 
this is the case. And whilst I confine myself, at present, to 
stating the matter suggestively, I may mention that in three 
localities very widely removed from each other, and in which 
the nature of the land is itself evidence of the liability or other- 
wise to a constantly running state of the water, this preponderance 
seems at all events undeniable. Thus on the borders of the 
Sunderbunds, where the whole country is a vast swamp, there 
are pools perpetually fed with fresh supplies of water, but only 
subject to currents during the inundations occurring during 
the rainy season. From such pools I obtained the most highly 
developed varieties of the symmetrical Diffiugian tests I have ever 
seen, namely, D. corona and D. lageniformis (figs. 2.¢ and 4c),— 
the oblique series and even D. spiralis being, however, moderately 
represented. At Goodhaab again, in West Greenland, I obtained 
material from pools occurring along the course of somewhat 
precipitous valleys, and accordingly under the constant action 
of the mountain streams by which these waters were supplied. 
* Fig. 3v gives the outline of a normal test of the pyriform variety 
of Diflugia proteiformis, whilst the dotted outline represents an ideal view 
of the same test made to assume a retort-shape by a force acting upon 
it laterally and in one direction. It is an instructive fact that the semicir- 
cular fold observable in the neck of this test and the neck of an ordinary 
retort are precisely similar; indeed the derivation of the word “retort ” 
explains the manner by which the curvature is effected in both cases. 
