34 DESMIDS OF THE UNITED STATES. 



Individuals swim free, scattered, or associated in gelatinous 

 masses ; cell membranes smooth or finely granulate, transparent or 

 fuscous, or reddish, often longitudinally striate. 



Multiplication take- place by division, and l>y zygospores 

 developed by the copulation of two cells. Coinp. Introduction. 



P. Digitus, (Ehrb.) Breb. Plate V, figs. 1,2. Plate LIII, fig. 1. 

 Cells ovately cylindrical, <>r broadly elliptical, 35 times 

 longer than broad, each end subtruncately rounded, i Plate 

 LIII. fig. I). Chlorophyl lamina lobate in the periphery, 

 interrupted in the middle. Plate V, fig. I. is a variety, more 

 elliptic than the typical form. Fig. 2 is a transverse section. 



Diameter 60- 80 u. 



This specie- appears to be widely distributed from Maine 

 t< Florida nnd wesl ward. 



P. lamellosum, Breb. Plate \'. tin. t. 



Cells oblong or fusiform cylindrical, often with a slight de- 

 pression in the middle, more or less attenuated towards the 

 obtusely rounded ends. Arrangement of chlorophyl usually 

 somewhat like the last, but not separated in the middle. 

 I tiameter 55 80 u. 



Not 90 common as the preceding. 



P. oblongum, I>. By. Plate V. fig. 17. 



( fells oblong cylindrical, 31 times as long a- wide, not con- 

 stricted in the middle, slightly tapering towards the ends; 

 aspices flatly rounded. Zygospore spherical, not unlike fig. 7 



I tind thi> form raostl) in fresh spring water, in gelatinous 

 gatherings. 



I', margaritaceum, Breb. Plate \'. figs. 5, 6, 11. 



Cylindrical, usually 8-9 times longer than broad, rarely, 

 only twice :i- long; not at all, or scarcely constricted in the 

 middle-, ends rounded, ornate with pearly granules arranged 

 in longitudinal series which often give a denticulate appear- 

 ance to the margin. 



Diameter 24-28 //.; length ordinarily about 225/*. I have 

 had specimens measuring 375 ". 



