NOTICE OF THli GENUS TETRAPEDIA, ETC. 361 



it is always somewhat strikingly evinced. I allude to a 

 gentle jerking or oscillatory movement of each cell — a sort of 

 quiet from-side-to-side vibration, generally, as if on a pivot 

 running through the isthmus, the radius of action being but 

 restricted, yet constant. Owing to this gentle movement not 

 being quite equable, in fact rather irregular, a change of 

 position of the cells occurs, but no noteworthy or decided 

 change of place. I Avould not be disposed to attribute any 

 great importance to this, yet it is a minor phenomenon, 

 which, in this little form, when met with, hardly ever fails to 

 arrest one's attention. This must, I should think, be 

 accounted a rare form, though it casually turns up in several 

 places, especially from county Dublin and county Wicklow. 

 I have not been able to sec any new growth or formation of 

 younger segments, nor any noteworthy difference in the 

 examples now and again offering themselves, save as before 

 mentioned, in dimensions. Bilaterality is thus the speciality 

 of this form. 



Even still less offering any tangible grounds upon which to 

 form a conjecture as to its mode of growth is the fourth form 

 in qu.cstion ; one, however, seemingly undoubtedly of a simi- 

 lar nature, and hitherto apparently unobserved (figs. 14-17). 

 This is also extremely minute, and is compressed, triangular, 

 the sides each with a single deep rounded sinus ; thus the 

 cell three-lobed, the angles broadly rounded, and each tipped 

 with an extremely slender linear bristle, in length almost 

 equal to or slightly longer than the diameter of the cell 

 itself. The contents are extremely pale ajruginous green. 

 Thus, the broad (or front) view might be comparable to the 

 end view of certain Staurastra, or even, perhaps, still more 

 aptly, it might be likened to a minute form of Polyedriuni 

 (Nag.), such as P. trigonum. But a second glance would 

 suffice to place beyond the smallest doubt that we had no 

 Staurastrum before us ; and, apart from the minute size, the 

 colour and aspect are abundantly characteristic to render it at 

 once decisive that this can be no Polyedriuni. No linear in- 

 cisions have ever been seen to occur, consequently the cells 

 do not present themselves as segmented, though forming 

 three equally divergent lobes, separated by the deep rounded 

 sinuses ; hence no example has been met with showing any 

 evidence of any mode of self-division. Their sizes vary, but 

 not so much as do those of the preceding. The most singular 

 feature is the possession of the very fine linear spine or bristle 

 at each ancfle or extremity of the lobes, inasmuch as an 

 appendage of such a nature would seem to be probably with- 

 out a parallel in Chroococcaceous forms, unless, indeed, the 



