222  Dr.G.C. Wallich on the Extent and Causes of ~ 
planation of the entire absence of young tests presenting the 
well-marked peculiarity of Difflugia spiralis, namely, that it is a 
modification of form produced by mechanical causes which only 
come into operation when the mitriform or acuminate test 
(figs. 2p, 3c, 36) approaches maturity. This may, however, 
be regarded as an exceptional case. But in D. corona, a variety 
found in tolerable abundance on the borders of the Gangetic 
Sunderbunds, and which is represented in figs. 44, 4c, I have, 
in like manner, failed to discover young tests in which the ere- 
nulated aperture is associated with the horn-like processes seen 
when it is mature. Medium-sized globular tests occur, how- 
ever, in which the crenulate aperture is fully developed (fig. 4a) ; 
and, on the other hand, mitriform specimens, such as those shown 
in fig. 38, in which a varying number of crenulations around the 
aperture is observable. Now there is nothing to distinguish the 
globular form with the crenulate aperture (fig. 4a) from the 
simple globular form without it (fig. 4), which constitutes one 
phase of D. proteiformis as given by Ehrenberg (Infusionsthier- 
chen, taf. 9. fig. 14), but the feature referred to; whilst the cre- 
nulate aperture occasionally reappears in the tuberculate variety 
of the latter form (fig. 4g). Lastly, individuals are frequently 
met with of the mature D. corona—that is, in which the horns 
are fully developed,—but exhibiting a perfectly plain aperture. 
Three conclusions are deducible from these facts,—the first 
being that neither the crenulation nor the cornua are constant 
in the variety named D. corona, singularly distinct though it 
appears if considered without reference to the osculant forms 
by which it is surrounded; the second, that there is nothing 
except the crenulation to distinguish the entire globular form 
with the crenulation (fig. 4a) from the universally distributed. 
globular variety (fig. 4A) ; and the third, that the crenulate mar- 
gin of the aperture is not even confined to the globular series of 
tests, but is to be seen occasionally in the mitriform varieties ; 
whilst neither in one set of forms nor in the other is there any- 
thing like constancy in the number of the crenulations them- 
selves. 
If we turn to the series of mitriform tests, of which the 
beautifully proportioned variety D. lageniformis (fig. 2c) would 
seem to be the culminating point, we find that the characters 
combining to make up this figure are gradually developed 
through the whole of the mitriform series, which has heretofore 
been subdivided, on the most trivial grounds, into four so-called 
species, namely, D. proteiformis, D. oblonga, D. acuminata (Khr.), 
and D. pyriformis (Perty). Thus, commencing from the earlier 
mitriform test, as shown in fig. 2, there is an unbroken transi- 
tion, both as regards extension in the longitudinal and trans- 
