Archer — the Genus Tetrapedia, 
305 
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 question — 
one, however, seemingly undoubtedly of a similar 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 aeruginous 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 
Polyedrium (Nag.), such as P. trigonum. But a second glance would 
suffice to place beyond the smallest doubt that we had no Staurastrura 
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 Polyedrium. No linear incisions have ever been seen to occur, con- 
sequently 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 angle or extremity of the lobes, 
inasmuch as an appendage of such a nature would seem to be probably 
without a parallel in Chroococcaceous forms, unless, indeed, the tuft of 
hair-like filaments surrounding the terminal "heterocyst" of a Cylin- 
drospermum may be something analogous. But there they are irregular, 
and more hair-like ; here solitary, definite in position, longer, and more 
like a very delicate linear spine, that is, not thicker at the base, or at 
any portion of its length. I have met with this organism on several 
occasions in moor-pools, chiefly in the counties of Dublin and Wicklow ; 
it is possible it may be pretty generally diffused, though not often seen, 
which may, indeed, in great part be due to. its great minuteness. 
Triradiate and setigerous are thus the special features of this form. 
Having thus tried to convey a conception of these four unicellular 
figured chroococcoids, the two first mentioned of which form Reinsch's 
genus Tetrapedia, as established by that author {op. cit,), it remains to 
consider in what manner the two latter, hitherto unrecorded, could be 
associated therewith. Taking the terms of the genus Tetrapedia, as 
founded, the two new forms would not correctly fall thereunder, the 
characters being based necessarily on the outward figure. If, indeed, it 
should be rigidly held that my two forms should be decidedly excluded 
from Reinsch's genus, in such case, fixing the generic limits on the same 
principle, each of these would seem to demand to be regarded as the 
types of two separate new genera — for to mj^self each would appear to 
be as distinct from each other as either from the two Reinschian forms.. 
R. I. A. PROC. — VOL, I., SER. II., SCIENCE. 2 li 
