302 
Proceedings of the Royal Irish Academy. 
interesting chroococcoid, T. gothica (Reinsch), so far as we are able to 
gather from the material afforded. The question is, what is its mature 
state ? Do any portions, more densely filled with contents become shut 
off, as resting-cells or " spores" (such as in Anabaina) ? What limit is 
there to this subdivision ? !Figure 6, with its sixteen quadrate seg- 
ments, offers at all the sides of all the segments the same minute 
central emargination, as in the earlier conditions, indicative (?) of a fur- 
ther formation of similar incisions, which, if carried out to the same 
extent, would result in each of the sixteen segments being cut into four 
further (tertiary) segments — such a tablet as fig. 6 represents, would 
then become one of sixty-four segments of four times the superficial 
dimensions of the former. Do the segments, at some epoch, and what, 
become cut through, and such as figs. 1 or 2 result ? The words of 
Eeinsch's description convey that this takes place, though he has 
nothing afterwards explanatory as to the point. Are the holes a con- 
stant characteristic ? or can they point to an impending dislocation or 
disuniting of the segments, thus initiated, in a singular manner, it is 
true, at a new place of origin, instead of by a carrying onwards of the 
vertical incisions ? The foramen or hole^ shown in fig. 5, resembles the 
central interspace of fig. 8, owing to the direction of its angles, but 
the holes in the compound tablet, shown in fig. 6, as before drawn 
attention to, have the angles in the reverse direction. They are de- 
picted as, and denominated holes, by Reinsch : may they be, after all, 
but interspaces, and the segments distinct cells, cohering at the adjacent 
angles by a gelatinous cushion (not united by a narrow isthmus of the cell 
itself, this afterwards perforated), and thus the opening amounting to no 
more than a break in the temporarily cementing medium ? But if, as we 
must infer as yet, they are truly openings formed in the " cell " itself, 
they do not seem to present themselves for some time, apparently at least 
until the primary incisions are advanced ; and the question arises as to 
how they first originate ? 
lior, indeed, are queries similar to those suggesting themselves, 
touching T. gothica, apparently at all more capable of receiving satis- 
factory replies, as regards any of the three other forms, constituting 
with it the subject of the present communication. The second form 
referred to this genus by Reinsch, Tetrapedia Crux-Michaeli, is (like the 
preceding) very minute, compressed, quadrate, the lateral margins 
entire, and, in all the examples noticed, with two shallow sinuses or 
concavities extending from the middle point of each side on to the 
angles, thus producing an obtuse- angled central protuberance, the 
margin at all the four angles deeply and obliquely incised, the incisions 
reaching to nearly two-fifths of the diagonal diameter of the cell, their 
sides rectilineal, acute at the inner extremity, slightly widening up- 
wards (fig .9). The incisions thus bisect the right angles of the 
quadrate cell, and subdivide it into four broadly cuneate segments, the 
outer angles of which are sub-acute, and with the two gentle sinuses, 
as before referred to, occupying the whole of the outer margin. The 
