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 subspeeific 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 3 u 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 i g^ro*^ 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 g to the inflated form of the same variety, which then 

 passes into the globular series (4 h), and through such forms as 

 2 a and 2 6 to the extreme variety of this series, namely D. lageni- 

 formis (2 c). 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. 3 a, 3 6, 3 c), and in fig. 3 s merging into 

 the variety called D. pyriformis. 



Fig. 4. Early stage of the globular series. Fig. 4 h represents the typical 

 subspecies D. globularis; fig. Ag, 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 homed 

 variety with the crenulate margin, D. corona. In this variety the 

 number of horns and also of crenulations varies considerably. 

 Finally, in fig. 4 A; we observe the incipient obUquity in the axis 

 of the globular form which suflBces 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. 6. Early state of the oblique series (subspecies) to which I have given 

 the name D. marsupiformis*. Figs. 5 a, 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. 5 m, a homed variety of the same, closely spproaching the 

 aculeate variety of the globular series (4 k). Figs. 5 b Sc 5 c, 



* Marsupium, a pouch. 



