Structural Variation among the Difflvgian Rhkopods. 221 



addition of mineral matter, if cemented to a point opposite tlie 

 aperture, may tend by its weight to depress the test in the di- 

 rection of its axis*, and thus give rise to the flattened series of 

 forms of which Difflugia aculeata is an example (fig. 4/;), and 

 Arcella vulgaris (Ehr. ) = Diffiugia vulgaris ("Wall.) is the extreme 

 limit; or cause it to assume the elongated cylindrical outline of 

 the mitriform series, of which such varieties as D. acuminata and 

 D. mitriformis are mature examples, should the particles of 

 mineral matter first employed be attached at points intermediate 

 between the aperture and the apex of the test (figs. 2, 2 p, q^ 3, 

 3 a, b, c, &c.). 



It is true that in Difiugia vulgaris (fig. 1 y) the test is nor- 

 mally depressed even in the entire absence of all foreign matter. 

 But it is also deserving of note that the most delicate and hyaline 

 tests of this species are those which present the greatest degree 

 of depression ; whereas the stouter and more deeply coloured 

 mature tests are those in which the hemispherical shape is most 

 j)erfectly maintained. Here then it is quite evident that 

 strength of the basal chitinoid material, as compared with the 

 weight it has to support, constitutes a mechanical condition upon 

 which the modifications of figure are dependent, and hence 

 lends confirmation to the view advanced with reference to Dif~ 

 flugia, even admitting, for the sake of argument, that Arcella 

 and Difflugia are generically distinct. 



Again, where the mineral matter is so uniformly distributed 

 over the young spherical test as to cause no deviation from its 

 normal outline, the globular or subglobular figure is maintained, 

 and attains its maximum development in such forms as are re- 

 presented by figs. 4 & 4 a-g. 



In another set of cases, in which the axis of the test may be 

 rendered oblique by a preponderance of mineral particles taking 

 place on one side, or through the pressure of a stream flowing 

 in one direction and thus acting upon the young test whilst yet 

 unconsolidated, the oblique or pouch-shaped series of forms is 

 produced, of which the simplest example is devoid of horn- 

 like appendages (figs. 5 a, 5 </) ; and a new variety, common in 

 many places, but most fully developed in Greenland, namely 

 D. cassis (figs. 5 6 & 5 c), may be taken as the extreme example. 



I have already given what I regard as the only probable ex- 



* By the axis of the test is meant the imaginary line which would pass 

 through the centre of the plane of the aperture and the apex. As the 

 pseudopodia are invariably projected through the aperture, the extremity 

 at which it occurs is called the anterior, the opposite being the posterior, 

 aspect. The apex of the test is the point furthest removed from the axis 

 of the aperture, whether reference be made to the symmetrically or 

 asymmetrically formed series. 



