RECENT MEMOIRS ON FRESHWATER RHIZOPODA. 201 



bring about the quite characteristic form of the test in the 

 two species known. Further, in reply to Dr. Wallich's objec- 

 tion as to Arcella vulgaris, with its remarkable and unique 

 test, how can one without violence refer such to one and the 

 same genus as those " stone-gathering " Difflugians above 

 referred to, not to speak of those others (still, indeed, 

 retained in Difflugia) with so remarkably differing tests 

 from those of T). piriformis, D. ohlonga. See, for example, 

 as those of D. spiralis, D. carinata, D. triangulata, &c. ? 



Dr. Wallich seems to think, in a letter with which he since 

 favoured me, that the remarks in my last communication, 

 which were mostly only a reproduction in English of Hertwig 

 and Lesser's, touching on the views so well known to be enter- 

 tained by him as regards the propriety of regarding Arcella 

 vulgaris as merely a form or species of the genus Difflugia, 

 were calculated to convey an erroneous impression of what 

 he really had expressed, or to indicate that he had not already 

 grasped, even considerably before the authors referred to, the 

 true structure in Arcella. Those authors were surprised that, 

 if he had so grasped the wide distinctions, rendering their 

 possessor, indeed, unique, he should still relegate Arcella 

 to Difflugia as a mere subform; and one would gather from 

 their remarks that they were inclined to fancy that from the 

 very fact of his doing so he could scarcely have truly realised 

 the distinctions after all. There can be no doubt indeed that 

 he did ; and I confess, for my part, that I feel surprised they 

 should appeal to him in vain. Whilst, indeed, I should 

 myself feel very sorry if, through oversight in any way, any 

 references of mine should seem, even remotely, to deprive 

 Dr. Wallich of the great credit which is his due, I can still 

 only wonder that observations so exact and experience so 

 great as his should have led him to views and conclusions 

 so divergent from those of most recent observers in this 

 field. 



Gromia paludosa, Cienkowski (fig. 5). 

 A seemingly remarkable form is described under the above 

 name by Cienkowski. He, however, characterises it as des- 

 titute of nucleus, which one may almost a ^jrion assume to 

 be erroneous. It seems otherwise from his description to 

 possess the general appearance and structure of a Gromia, but 

 the considerably elongate and pronounced pseudopodial stem 

 is not apical and in a line with the axis, but lateral. The 

 test is delicate and colourless, oviform, with but a very 

 slightly elongate "neck." The pseudopodial stem takes 



' Loc. cit., Bd. xii, p. 32, t. vi, f. 41-47. 



