DESMIDITM. 27 



spherical form ; this enlarges, and as it increases in size, 

 forces the sides of the cell apart to more than twice the origi- 

 nal diameter (fig. 3) ; when mature the Sides separate and the 

 spores are set free. 



D. <)UADRANGULATUM, Kg. Plate II, figs. 13, 14. 



Filament quadrangular, varying in breadth from its twist- 

 ing, having two longitudinal waved lines ; joints in front 

 view broader than long, with two somewhat rounded crena- 

 tures on each lateral margin. End view quadrangular. 



Diameter 50-60 ^. 



This is a form which prevails in England and on the Con- 

 tinent, but has not yet been found in this country. I insert 

 it from Dr. Wood's contribution in anticipation of yet find- 

 ing it. 



D. APTOGONIUM, Breb. Plate II, figs. 6, 7. Plate LX, fig. 7. 



Joints in front view, quadrangular, broader than long, 

 with two rounded crenatures on each lateral margin, united 

 at the outer portion only of each end margin by mutual pro- 

 jections, thus producing intervening central oval or oblong 

 foramina. 



Diameter 25-38 n- 



Not so frequent as D. Swartzii, but it appears to be widely 

 scattered from Pennsylvania and New Jersey as far south as 

 Florida, and probably also northward and westward. 



D. BAILEYI (Aptogonum Batteyi, Ralfs). Plate II, figs. 8-12. 



Filaments triangular, twisted as the preceding, joints 

 united by each end of the lateral margins only, excavated 

 between the angles as the preceding form ; the lateral mar- 

 gins are straight, not bicrenate, a characteristic which sepa- 

 rates it from the other species. 



Diameter 25 /*., more or less. 



Found frequently in sluggish waters, without being con- 

 fined to any particular State or States. 



The process of fruiting is not unlike that of D. Swartzii, 

 the separated portions of chlorophyl (fig. 8) unite and concen- 

 trate (fig. 9), then enlarge (fig. 10), press out the sides to more 

 than twice the diameter of a sterile filament, then break 

 apart and set the spores free. 



