Structural Variation among the Difflugian Rhizopods. 223 



verse axis of the test^ to the broadly ovate shape shovy^n in fig.Zy, 

 and the almost cylindrical varieties which have been termed D. 

 ohlonga or D. acuminata, as they happen to present the acumi- 

 nate or simply convex outline posteriorly (figs. 3 ^ & 3 6) ; whilst, 

 by a compromise, as it were, between such shapes as are depicted 

 in figs. 3 / & 2 g-, we have the balloon-like or pear-shaped va- 

 rieties (figs. 26 &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. 2 a. But this is by no means confined to one 

 form of test ; it is to be seen now and then in the apei'ture of 

 such forms as figs. 2q, 4>h, ^g, and indeed in all the varieties 

 of the forms represented in figs. 2 i, 4, & 4ig, and depends on 

 the eversion of the marginal border of the basal chitinoid layer 

 80 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 lip which becomes so largely developed in the extreme 

 variety of the series, namely, D. lageniformis (fig. 2 c). 



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 mineral 

 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 



