226  Dr.G.C. Wallich on the Extent and Causes of 
acuminate variety of D. mitriformis (fig. 3c) ; whilst the second 
consists in a partial inversion of the apertural lip, instead of an 
eversion, as occurs in D. lageniformis (fig. 2c). But the chain 
of forms is rendered still more complete; for individuals are 
now and then met with, of the mitriform series (D. acuminata), 
with a couple of cornua placed on each side on the actual apex 
of the test (Plate XVI. fig. 8); so that between the mitriform 
and the oblique varieties under notice the compressed and oblique 
figure and commencing inversion of the lip constitute the sole 
difference. 
Lastly, we arrive at the extreme limit of the oblique series, in 
which, however, there are no cornua; but excentricity of the 
aperture becomes greatest, and its entire margin is inverted so 
as to constitute a short tube. extendmg upwards into the cavity 
of the test. This last character, singular as it appears, has its 
counterpart in the Entosolenian group of the Lageniform Fora- 
minifera; and I am glad of the opportunity of stating that the 
generic separation of Entosolenia from Lagena is sisted on by 
Professor T. R. Jones and Mr. Parker for the same reasons 
that are here advanced, namely, that the character indicates 
only varietal distinction (‘Introduction to the Study of the Fo- 
raminifera, by Dr. Carpenter, pp. 157, 158). 
The very singular helmet-shaped variety (figs. 5 6, 5 c), which 
may be regarded as the antithesis to D. lageniformis, is repre- 
sented as it occurs in the Greenland material. In it we merely 
perceive, in its most exaggerated degree, the obliquity already 
so marked in the variety shown in fig. 5 m, the depression of 
the test, together with the inversion of the margin of the aper- 
ture, occurring in the early form shown in figs. 5 & 5 m, clearly 
proving howthe latter connects D. cassis with the globular varieties. 
One more transition in the Difflugian series remains to be 
noticed before I conclude this section of my subject. It is an 
_ important one, however, since it seems clearly to indicate that, 
whilst the animals of Difflugia and Arcella are generically iden- 
tical, there is no such difference between their respective tests as 
can constitute more than a subspecific separation. 
In the Difflugian test we constantly witness a structure similar 
to that first pomted out by Messrs. Jones and Parker as per- 
vading the group of Foraminifera to which they have assigned 
the name of Lituolide. But we must bear in mind one point of 
difference—namely, that whereas in the Lituoline group the mo- 
dification of material employed in the construction of the shell 
entirely supplants the normal calcareous material which is se- 
creted by the animal, amongst the Difflugie there is no normal 
mineral secretion to supplant*. I allude to the composition of 
* Tam aware that the word “secrete” expresses more than we have 
