Structural Variation among the Difflugian Rhizopods. 243 
singularly fitted to throw light on the laws which regulate the 
variation of species. For, however true it is that these lowest 
animal forms are prone to variation in an unprecedented degree, 
this cannot surely be advanced as a plea for assuming that the 
laws which govern specific variation amongst the higher orders 
of creation must necessarily be of a distinct nature. 
Kensington, Feb. 19, 1864. 
EXPLANATION OF THE PLATES. 
PLATE XV. 
This Plate is designed to show the order in which the four subspecies of 
Difflugia proteiformis arrange themselves around a common archetypal or 
embryonic centre; and their several varieties may be supposed to pass 
transitionally from one subspecific type to the other. It may be mentioned 
that none of the figures in either of these plates are diagrammatic, except 
the dotted half of figure 3v in Plate XV.; and, with the exception of the 
central figure (fig. 1) of Plate XV., which is magnified about 800 diameters, 
that the whole of the other figures in both Plates are magnified from 
200 to 300 diameters. 
Fig. 1. Embryonic test of Difflugia from which the entire series take their 
origin, its diameter being about 5 4';5th of an inch, and the aper- 
ture formed by the truncation of from }th to {th of the diameter 
in one direction. 
Fig.2. The early state of the variety D. mitriformis, passing through 
2p and 2¢q to the inflated form of the same variety, which then 
passes into the globular series (4 /), and through such forms as 
2a and 26 to the extreme variety of this series, namely D. lageni- 
formis (2c). In this form the eversion of the lip of the aperture 
attains its maximum limit. 
Fig. 3. Another young test of the mitriform series, passing in one direc- 
tion into D. spiralis (3 v), in another exhibiting the various forms 
of D. acuminata (figs. 3a, 36, 3¢), and in fig. 3s merging into 
the variety called D. pyriformis. 
Fig. 4. Early stage of the globular series. Fig. 4h represents the typical 
subspecies D. globularis; fig. 4g, the allied variety D. tubercu- 
lata; whilst fig. 4 a@ shows the occurrence of a crenulate aper- 
ture in the typical form, and, hence, the transition to the horned 
variety with the crenulate margin, D. corona. In this variety the 
number of horns and also of crenulations varies considerably. 
Finally, in fig. 4k we observe the incipient obliquity in the axis 
of the globular form which suffices to render the position of the 
aperture excentric, and the horns met with in D. corona, but 
generally (as seen here) occupying only one-half of the test. 
This is the form referred to by Ehrenberg as Arcella aculeata. 
Fig. 5. Early state of the oblique series (subspecies) to which I have given 
the name D. marsupiformis*. Figs. 5a, 5d, 5e are front and 
side views of the plain variety, showing great excentricity of the 
aperture, great vertical depression, and the inversion of the lip. 
Fig. 5m, a horned variety of the same, closely spproaching the 
aculeate variety of the globular series (4k). Figs. 56 & 5c, 
* Marsupium, a pouch. 
