Sh-uctural Variation among the Difflngian Rhizopods. 225 



In these the most highly developed forms were the ohlique ones, 

 — D. cassis (which may be regarded as the limit of this series, 

 as I have already shown) attaining its extreme characters (figs. 

 5 b, 5 c), and, what must be considered as equally significant, 

 the tests of the larger oblique varieties, and also of the common 

 mitriform scries, being loaded to an extraordinary degree with 

 mineral matter, chiefly in the shape of diatoms, doubtless to 

 render their weight greater (Plate XVI. fig. 9). 



Lastly, in a locality close at hand, namely Hampstead, in little 

 pools extending down the slopes and perpetually subject to a 

 dribbling stream, the oblique varieties are abundant, though 

 not attaining the extreme characters of D. cassis — such varieties 

 as D. aculeata and D. spiralis being common ; whilst the tests 

 of the mitriform series are frequently covered with masses of 

 mineral matter so large in proportion to the entire size of the 

 tests, and so irregular, as to render it far from improbable that 

 their weight and outline must exercise some power in enabling 

 them to hold their ground. 



But, to revert to the varietal development of the series now 

 under notice, taking the globular young test once more as a 

 starting-point, we find an extremely gradual transition taking 

 place, first till we arrive at full-sized mature tests the spherical 

 outline of which is only disturbed to the extent of making the 

 aperture appear slightly excentric. This variety is represented 

 in fig. 4 k. In the latter example, however, we perceive the hol- 

 low horn-like processes which have been regarded as indicating 

 the species to which the name of D. aculeata {Arcella aculeatOy 

 Ehr.), has been given. Now there is not a single character to 

 distinguish a variety of this form without horns from the common 

 globular variety (fig.4A), save this trifling obliquity or compres- 

 sion. On the other hand, there is nothing to separate the horned 

 variety (fig. 4 k) from D. corona when devoid of the crenulated 

 aperture, but this same obliquity ; for although the cornua are 

 generally distributed only over that half of the test furthest re- 

 moved from the aperture, this peculiarity is occasionally met on 

 the side ofi). corona by a similar asymmetrical disposition of its 

 cornua ; whilst specimens are now and then to be found of 

 Z). aculeata in which the cornua really form a complete circlet, 

 but, owing to the tendency of the test to rest in a position per- 

 pendicular to the plane of the aperture, the part anterior to the 

 aperture prevents a certain portion from being easily seen. 



In the variety shown in figs. 5 a, </, & e, we have the transition 

 from the plain, small, oblique test (fig. 5) to the form of D. aculeata 

 shown in fig. 5 m. But two slight peculiarities now make their 

 appearance, — the first being that the horns become longer and 

 identical with the broad-based apical horn sometimes seen in the 



