80 Mr. H. J. Carter on Freshwater Rhizopoda 



part, the anterior half being rather contracted, and the aperture 

 large and cii-cular, or slightly elongated transversely. The latter 

 feature, together with the tendency of the aperture to an ex- 

 centric or terminal position and the depression of the test ante- 

 riorly, while it is elevated behind, causes this Rhizopod to bear 

 a similar relation to Difflugia that the pleurostomatal form of 

 Euglypha [Trinema acinus, Duj., Euglypha pleurostoma, Cart.) 

 bears to the latter genus ; and when we observe that its test is 

 formed of grains of hyaline quartz, and that the pseudopodia 

 arc digital and obtuse, the alliance is still greater. The spines 

 then become the chief distinguishing character, out of which, 

 according to Claparede and Lachmann (p. 447), delicate pseudo- 

 podal prolongations are projected. But even the spines vary 

 greatly in number and position {i, d), while in some varie- 

 ties they are altogether absent {ff, h), as may be seen by the 

 group of figures I have given of them, which, being all drawn 

 upon the same scale, show not only their relative sizes to each 

 other, but to those of the other Difflugice. 



In the lateral view of the large specimen [d), I have endea- 

 voured to give another variety in the disposition of the spines, 

 and have figured on the test the regular arrangement of short 

 straight filaments (e) which, at Bombay, I found occasionally 

 substituted for the grains of quartz ; while in other instances the 

 test was composed of a mixture of both, and sometimes of the 

 frustules of Navicula pusilla (mihi) only, with the endochrome 

 still in them. Thus, as we have now seen, these animals avail 

 themselves of much variety in the material of their covering, 

 although grains of sand, and especially of colourless hyaline 

 quartz, seem to be preferred. 



Arcella, Ehr. 

 Arcella vulgaris, Ehr. PI. II. fig. 14. 

 Among the figured specimens of this Rhizopod and its varie- 

 ties are the largest that I have seen ; and in the horizontal view 

 of the depressed form will be observed the double nucleus, viz. 

 one situated diametrically opposite the other. This occurs also 

 in all the varieties; and I wonder that no allusion has been 

 made to the circumstance by other authors who have written on 

 this Rhizopod, especially since my figure illustrating the fact 

 was published in 1856 (Annals, ser. 2. vol. xviii. pi. 7. fig. 79). I 

 think that I have found specimens with only one nucleus, but 

 certainly none with " twelve or fifteen," as stated by Auerbach 

 and repeated by Claparede and Lachmann (p. 445). But I 

 have figures of specimens, observed in the island of Bombay, 

 containing even more than this number of what appear to me 

 now to have been "reproductive cells" analogous to those which 



