Structural Variation among the Difflugian Rhizopods. 225 
In these the most highly developed forms were the oblique ones, 
—D. cassis (which may be regarded as the limit of this series, 
as I have already shown) attaining its extreme characters (figs. 
5 6, 5c), and, what must be considered as equally significant, 
the tests of the larger oblique varieties, and also of the common 
mitriform series, being loaded to an extraordinary degree with 
mineral matter, chiefly in the shape of diatoms, doubtless to 
render their weight greater (Plate XVI. fig. 9). 
Lastly, in a locality close at hand, namely Hampstead, in little 
pools extending down the slopes and perpetually subject to a 
dribbling stream, the oblique varieties are abundant, though 
not attaining the extreme characters of D. cassis—such varieties 
as D. aculeata and D. spiralis being common; whilst the tests 
of the mitriform series are frequently covered with masses of 
mineral matter so large in proportion to the entire size of the 
tests, and so irregular, as to render it far from improbable that 
their weight and outline must exercise some power in enabling 
them to hold their ground. 
But, to revert to the varietal development of the series now 
under notice, taking the globular young test once more as a 
starting-point, we find an extremely gradual transition taking 
place, first till we arrive at full-sized mature tests the spherical 
outline of which is only disturbed to the extent of making the 
aperture appear slightly excentric, This variety is represented 
in fig. 4k. In the latter example, however, we perceive the hol- 
low horn-like processes which have been regarded as indicating 
the species to which the name of D. aculeata (Arcella aculeata, 
Khr.), has been given. Now there is not a single character to 
distinguish a variety of this form without horns from the common 
globular variety (fig.4:/), save this trifling obliquity or compres- 
sion. On the other hand, there is nothing to separate the horned 
variety (fig. 44) from D. corona when devoid of the crenulated 
aperture, but this same obliquity ; for although the cornua are 
generally distributed only over that half of the test furthest re- 
moved from the aperture, this peculiarity is occasionally met on 
the side of D. corona by a similar asymmetrical disposition of its 
cornua; whilst specimens are now and then to be found of 
D. aculeata in which the cornua really form a complete circlet, 
but, owing to the tendency of the test to rest in a position per- 
pendicular to the plane of the aperture, the part anterior to the 
aperture prevents a certain portion from being easily seen. 
In the variety shown in figs. 5 a, d, & e, we have the transition 
from the plain, small, oblique test (fie. 5) to the form of D. aculeata 
shown in fig. 5m. But two slight peculiarities now make their 
appearance,—the first being that the horns become longer and 
identical with the broad-based apical horn sometimes seen in the 
