HARPER: CELL TYPES AND RESPONSES IN PEDIAsTRUM 229 
question of the relation of the larger cell numbers to the con- 
figuration of the colonies in another paper. The shortness and 
form of the spines suggest in some degree those of P. Ehrenbergii, 
but the species cannot be regarded as in any proper sense a transi- 
tion form between the Diactinia and Tetractinia. 
D. Pediastrum tricornutum Borge. 
This species, representing perhaps a series of Triactinium 
(Nitardy makes it Diactiniopsis), I have not seen. It differs 
characteristically from other types in that the three spines do not 
lie in the same plane and hence have no part in determining the 
intercellular contacts in the colony. Only eight-celled colonies 
have been figured so far—one cell in the center surrounded by 
seven. Such a form could hardly be conceived as developing 
from the Diactinia by progressive variation. It may have 
originated from a form like P. integrum as a representative of 
quite a different line of development or it may have connections 
with Coelastrum in quite a different series. 
E. Tetractinium.— Pediastrum Ehrenbergii A. Br. 
The Tetractinia illustrated by the common Р. Ehrenbergii 
(FIGS. 23, 24, 25, 26) are those types in which the two spines or 
Fics 23, 24, 25, and 26. Pediastrum Ehrenbergii A. Br., with varying degrees 
of lobing of the cells. Figure 26 shows a four-celled colony with one cell almost 
free but showing none the less the characteristic wedge-shaped form, XX about 350. 
Fig. 23 X about 1000, Fig. 24 X about 700, Fig. 25 X about 550. 
lobes of the Diactinia tend to become more or less deeply bifid. 
The species commonly occur in four-, eight-, and sixteen-celled 
