EXPLANATION OF THE PLATES. 333 



birth.) The emargination of the border-cells has passed 

 into the formation of horns. The cells are not even yet 

 closely connected together, but exhibit spaces between 

 them, so that in this stage the disk exhibits a resem- 

 blance to that of Pediastrum pertusum, (Nag., ' Einz. 

 Alg.,' V. 3, as P. Selenaea,) which species, however, does 

 not lose the orifices in the full-grown condition. Not 

 until the second day, after an interruption during the 

 night, of the rapid changes of form of the cells, do the 

 cells become closely applied together ; the horns acquire 

 their proper shape and length at the same time. 



Pig. 10. A half-grown disk of four cells, two of which 

 meet in the middle. A starch-grain is visible in each 

 cell, as is usual in the middle age. The mother-vesicle 

 is still visible here, while in ordinary cases it disappears 

 altogether by the second day. 



Fig. 11. An older four-celled disk, the four cells 

 meeting in the middle. The families formed of four 

 cells are extremely rare in this species. 



Pig. 12. Disk of eight cells, in the arrangement 2+6, 

 which is more frequent in this species than the arrange- 

 ment 1+7. The inner two cells are notched on the 

 outer border, which is connected with the position of the 

 two outer cells opposite to them. 



Fig. 13. A similar disk, but with the inner two cells 

 not notched, but interposing an obtuse-angled prolonga- 

 tion into the commissure of the outer cells alternating 

 with them. 



