Structural Variation among the Difflugian Rhizopods. 241 



Subspecies 2. D. globularis (Duj.). 



Characters. Test more or less globular, from ^th to ^t\\ 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. hthercuJata. Surface of test covered with subhemispherieal 

 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. /3. 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 piano- 

 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 or narrowest portion of 

 the test. In the/row^ 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. at. 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 DiffiugidcB 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 



