216 SEMI-CENTENNIAL OF ToRREY BoTANICAL CLUB 
figures likewise only one symmetrical sixteen-celled colony (93, 
pl. 19. f. 9) against five which are irregular, but the form of the 
cells in all of them is essentially the same. 
Nitardy (14) has had an abundance of the more symmetrical 
forms and their variants which have been variously named simplex, 
duodenarium, etc., and, as noted, puts them under the name P. 
triangulum (Ehrenb.) A. Br. with two varieties, angustum and 
latum. The latter in its cell form (Nitardy, pl. 8. f. 5) agrees 
with the forms I have found and the two varieties seem fairly 
well marked in the case of his figures 3 and 5, plate 8. In some 
of his other figures the distinction is not so clear, but the drawing 
is rather crude and it is hard to judge. In his variety angustum 
Fics. 2, 3, and 4. Pediastrum simplex Meyen, sixteen-celled colonies showing 
the cell arrangement 5 + rx and 4 + 12, X about 175. Figure 4 shows one of the 
central cells with a well-developed spine, pointing downward, but not very clearly 
down in the reproduction. 
the lobes of the cell are more slender and form large intercellular 
spaces. The sixteen-celled colonies may have a very definite bi- 
laterally and radially symmetrical arrangement of their cells with 
four in the center in a square, surrounded by four groups of three 
(Nitardy, "14, pl. 7. f. 5), or two groups of four and two groups of 
two (Nitardy, 714, pl. 9. f. 20), or five in the center, a pentagon 
surrounded by eleven—four pairs and three (Nitardy, 714. pl. 5. 
f. 1). These symmetrical figures are from his own observations 
on material from Grünewaldsee near Berlin and are very fine 
illustrations of the capacity of the swarm-spores of this species 
to achieve delicately balanced equilibrium relations such as are 
necessitated by the one-spined cell form. I have not seen these 
symmetrical forms and have taken from Nitardy (714) FIGURES 
27, а, b, etc., to illustrate the very interesting bilateral and radial 
symmetry which these sixteen-celled colonies may show. 
