Structural Variation among the Difflugian Rhizopods. 241 
Subspecies 2. D. giobularis (Duj.). 
Characters. Test more or less globular, from 4th to 3th being 
truncated and forming the aperture. Margin of the latter plain or 
crenulate. Test occasionally furnished with cornua, sometimes 
arranged symmetrically, sometimes the reverse ; their number vari- 
able. Test occasionally depressed vertically and excentrically. 
Var. «. D. tuberculata. Surface of test covered with subhemispherical 
nearly equal-sized elevations, which give it a mulberry-shape. 
In some tests there are corresponding hollows on the interior 
of the chitinoid wall. 
Var. 8. D. aculeata. Test generally compressed excentrically, so that 
the aperture is also excentric. Margin of aperture generally 
inverted to some extent, but not always. Surface of test either 
furnished with a varying number of finger-like cornua, or plain. 
The cornua for the most part arranged in a halfcircle round 
the posterior half of the test; but their position, as well as 
shape, is very variable. 
Var. y. D. corona. Test crown-shaped, furnished with from three to 
eight conical cornua placed around its posterior third. Margin 
of aperture regularly crenulate; number of crenulations 
variable. 
Subspecies 3. D. arcella (Ehr.). 
Characters. Test chitinoid, rarely if ever presenting mineral mat- 
ter on its surface, which is studded with regular but very minute 
hexagonal reticulations. Form presenting varying degrees of plano- 
convexity, the convexity at times amounting to that of a hemisphere; 
the aperture small, invariably occupying the centre of the plane sur- 
face, and its margin being more or less inverted. 
Subspecies 4. D. marsupiformis (Wall.). 
Characters. Test varying, in the side view, from that of a slightly 
excentric and depressed hemisphere to the pyriform outline, but with 
this peculiarity, that the aperture, as seen in this view, occupies a 
position on one margin of the pyriform figure, and extends from the 
centre to a point approaching the anterior cr narrowest portion of 
the test. In the front view, the test is pouch-shaped. Margin of 
aperture markedly inverted. The mature test generally presents 
from three to six cornua arranged over its posterior third. 
Var. «. D. cassis (Wall.). Merely a small and extreme variety of the 
marsupiform test, but rarely presenting the cornua. 
On comparing the above arrangement and characters of 
the Difflugide with the figures in the accompanying plates to 
which attention is directed, there will, I presume, be but little 
difficulty in recognizing the points from which the predominant 
groups of varieties take their origin, and in tracing the grada- 
tions passed through by the most aberrant forms. I have 
deemed it preferable to represent the groups as diverging from 
