Structural Variation among the Difflugian Rhizopods. 228 
verse axis of the test, to the broadly ovate shape shown in fig.2¢, 
and the almost cylindrical varieties which have been termed D. 
oblonga or D. acuminata, as they happen to present the acumi- 
nate or simply convex outline posteriorly (figs. 3 ¢ & 36); whilst, 
by a compromise, as it were, between such shapes as are depicted 
in figs. 3¢ & 2q, we have the balloon-like or pear-shaped va- 
rieties (figs. 2b & 3s). 
Again, taking the young test (fig. 2) as a starting-point, we 
occasionally meet with what, at first sight, appears to be a well- 
defined narrow but plain band surrounding the aperture, as 
shown in fig. 2a. But this is by no means confined to one 
form of test; it is to be seen now and then in the aperture of 
such forms as figs. 2g, 44, 4g, and indeed in all the varieties 
of the forms represented in figs. 27, 4, & 4g, and depends on 
the eversion of the marginal border of the basal chitinoid layer 
so that it overlaps the margin of the foreign particles impacted 
on the outer surface of the test. In short, this is the rudiment 
of the dip which becomes so largely developed in the extreme 
variety of the series, namely, D. lageniformis (fig. 2 ¢). 
Now the crenulate margin of D. corona and the other varie- 
ties that present this character is merely a modification of the 
apertural band just referred to, in which the margin of the basal 
chitinoid layer, instead of being unbroken, is formed into a series 
of minute crenulations, the points of which alone reach the ex- 
treme edge of the aperture, without apparent eversion, however. 
We can hardly fail to perceive that this arrangement of the 
apertural margin of the Difflugian test is evidently the best 
fitted to admit of the easy projection and retraction of the glairy 
pseudopodia. I believe it pervades the entire series of varieties, 
although more readily, or at all events more generally, recogniz- 
able in certain forms. In all we can perceive that the greatest 
care seems to be lavished on the selection of minute mmeral 
particles for deposition immediately around the aperture; and, 
as I shall presently show, that in those examples in which there 
is no admixture of extraneous mineral matter, but the entire test 
is made up of chitinoid substance, there is evidence of a like 
degree of adaptative power. Were the evidence otherwise in- 
complete, I conceive, however, that the structure of the beautifully 
moulded lip of D. lageniformis would prove conclusive ; for in 
it we find that whilst the mineral particles with which the rest 
of the test is as it were tessellated extend to the commencement 
of the everted portion, from that point they gradually dwindle 
away, until at last it is only with considerable care in the ad- 
justment of the light and focus that we are enabled to perceive 
the almost hyaline margin of the overhanging lip. 
Lastly, we arrive at the singular group of forms in which the 
