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 Diffiugia 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, Sc, 3 6) 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. 4 6, 4 c, I have, 

 in like manner, failed to discover young tests in which the cre- 

 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. 4 a) ; 

 and, on the other hand, mitriform specimens, such as those shown 

 in fig. 3, 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. 4 a) from the 

 simple globular form without it (fig. 4 A), which constitutes one 

 phase of D. proteiformis as given by Ehrenberg (Infusionsthier- 

 chen, taf. 9. fig. lb), but the feature referred to; whilst the cre- 

 nulate aperture occasionally reappears in the tuberculate variety 

 of the latter form (fig. 4^). 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. 4 a) from the universally distributed 

 globular variety (fig. 4 h) ; 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. 2 c) 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 (Ehr.), 

 and D. pyrifonnis (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- 



