Archer — On the Genus Tetrapedia. 
303 
edge or side-view of the cell presents a lanceolate outline, the extremi- 
ties acute (fig. 10). 
The quadripartite condition is regarded by Reinsch as a " state of 
division;" but he does not figure or refer to the simple or undivided 
state : it is not clear, indeed, how the carrying onwards of the incisions 
to completion, even until it should involve entire separation of the seg- 
ments, would result in the formation of a secondary set of quadrate 
cells, like the assumed young form — if pushed through and through the 
quadrate cell would simply become cut into four triangular ones — two 
sides straight and one with the two sinuses. 
One might suggestively, however, put the following possibility. 
Assuming the incisions carried on till four distinct younger cells were 
produced, of course of the triangular figure, they each would be equal 
to one-fourth the superficial dimensions of the original cell, and 
they would resemble a moiety of a similar cell of one-half its superficial 
dimensions, and, as if separated from its assumed fellow diagonally. 
N'ow, we might suppose each of them capable of growing a new half of 
triangular form, the obtuse-angled prominence taking the place, as it 
were, of a kind of punctum vegetationis^ that is (so to say), as if the cell 
were stretched or pulled by some force to double its size, the little pro- 
minence always remaining the apex, until a quadrate figure were 
attained ; it would, of course, likewise be requisite to suppose a general 
increase in size of the whole cell thereupon, ov pari passu; they occur, 
however, of varying sizes. It would further be necessary to suppose a 
new incision to originate at each angle, and the central protuberance 
to be developed at each side, producing the two sinuses. Were all this 
accomplished in each of the separated cuneate segments, four new simi- 
lar cells to that figured would be the result. This, however, is but 
quite conjectural, and it seems to remain quite uncertain as to how the 
cells grow and divide into new cells. The few examples I have seen 
were similar to those figured, that is, deeply incised with the four 
broadly cuneate segments, the specimens slightly differing in dimensions, 
and showed no tendency to any further alteration. There is a certain 
amount of possibility of this form being taken for a minute Pediastrum 
(Meyen); but apart from the colour of the contents, there is no Pedias- 
trum with similar outline of the cells, and, indeed, as regards the ex- 
amples hitherto seen, the common union of the segments at their bases 
prevents any confounding with a Pediastrum, which is composed of 
distinct but closely juxtaposed cells, held together as a frond or 
coenobium" by a kind of hyaline intercellular substance." Obliquely 
quadripartite is thus the great feature of this form. 
JN'ot less problematic, on all points, is the third form (figs. 11,12). I 
would draw attention to one, which, so far as I can learn, has not 
hitherto met observation it is clearly, however, one of kindred nature 
* During the interval that the above Paper was in the hands of the printer, I 
enjoyed the great gratification and valued privilege of making the personal acquaint- 
ance of Dr. Veit B. Wittrock, of the University of Upsala, on the occasion of a (too 
