Structural Variation among the Difflugian Rhizopods. 221 
addition of mineral matter, if cemented to a point opposite the 
aperture, may tend by its weight to depress the test in the di- 
rection of its axis*, and thus give rise to the flattened series of 
forms of which Difflugia aculeata is an example (fig. 44), and 
Arcella vulgaris (Ehr.) = Difflugia vulgaris (Wall.) is the extreme 
limit ; or cause it to assume the elongated cylindrical outline of 
the mitriform series, of which such varieties as D. acuminata and 
D. mitriformis are mature examples, should the particles of 
mineral matter first employed be attached at points intermediate 
between the aperture and the apex of the test (figs. 2, 2 p, q, 3, 
od, b,c, &e.). ° 
It is true that in Difflugia vulgaris (fig. 1 y) the test is nor- 
mally depressed even in the entire absence of all foreign matter. 
But it is also deserving of note that the most delicate and hyaline 
tests of this species are those which present the greatest degree 
of depression ; whereas the stouter and more deeply coloured 
mature tests are those in which the hemispherical shape is most 
perfectly maintained. Here then it is quite evident that 
strength of the basal chitinoid material, as compared with the 
weight it has to support, constitutes a mechanical condition upon 
which the modifications of figure are dependent, and hence 
lends confirmation to the view advanced with reference to Dif- 
jlugia, even admitting, for the sake of argument, that Arcella 
and Difflugia are generically distinct. 
Again, where the mineral matter is so uniformly distributed 
over the young spherical test as to cause no deviation from its 
normal outline, the globular or subglobular figure is maintained, 
and attains its maximum development in such forms as are re- 
presented by figs. 4 & 4: a-g. 
In another set of cases, in which the axis of the test may be 
rendered oblique by a preponderance of mineral particles taking 
place on one side, or through the pressure of a stream flowing 
in one direction and thus acting upon the young test whilst yet 
unconsolidated, the oblique or pouch-shaped series of forms is 
produced, of which the simplest example is. devoid of horn- 
like appendages (figs. 5 a, 5d); and a new variety, common in 
many places, but most fully developed in Greenland, namely 
D. cassis (figs. 5 b & 5c), may be taken as the extreme example. 
I have already given what I regard as the only probable ex- 
* By the axis of the test is meant the imaginary line which would pass 
through the centre of the plane of the aperture and the apex. As the 
pseudopodia are invariably projected through the aperture, the extremity 
at which it occurs is called the anterior, the opposite being the posterior, 
aspect. The apex of the test is the point furthest removed from the axis 
of the aperture, whether reference be made to the symmetrically or 
asymmetrically formed series. 
