106 FRESH-WATER ALG^ OF THE UNITED STATES. 



P. liying on rocks and mosses, swimming in a transparent, sometimes light-green jelly ; cells 

 obtusely truncated, rounded at the ends, 2-3 times longer than broad ; chlorophyl lamina 

 axillary, mostly indistinct, often wanting ; endochrome light-green ; nucleus generally distinct; 

 zygospore subfuscous, either globose or of an irregular form, somewhat resembling that of an 

 hour-glass ; external coat irregularly excavated and sulcata. 



Remarks. — This species was found along the North Pennsylvania Kailroad, near 

 Chelten Hills, growing amid mosses on the rocky juttings over which the water 

 was dripping. It occurs as a rather firm, transparent jelly, mostly of a light 

 greenish tint, in which the cells are often placed quite thickly. They are cyUn- 

 drical, mostly straight, but sometimes slightly curved, and often completely filled 

 with a light greenish endochrome. The central lamina is irregular, and mostly 

 not at aU pronounced. In some cells the endochrome is much broken up, so that 

 the interior is filled with little green masses with light spaces between them. In 

 these cells the nucleus is generally not perceptible, whilst in the others it is very 

 weU marked. The zygospore is often globular, sometimes it is irregularly elliptical, 

 with a constriction in the centre, so as to give it somewhat of an hour-glass shape. 

 The outer coat mostly fits pretty closely on the inner contents, and is very often 

 distinctly marked with little pits, some round, some irregular in shape; in other 

 cases, instead of being thus pitted, the spores seem to be marked with deep curved 

 furrows. 



Fig. 4, pi. 11, represents this plant in different stages of growth. (See Expla- 

 nation of Plates.) 



Genus PENIUM, Breb. (1848.) 



Cellulse cylindricae vel fusiformes, rectae, utroque polo rotundatse vel truncato-rotundatss (nee emar- 

 ginatse nee excisse), medio saepius constrictse. Lamina chlorophyllacea axilis, ex transverso conspecta 

 radiatim-divergens, radii saepe furcati, granula amylacea plerumque longitudinaliter seriata includens. 

 Individua in aqua libere natantia, singula, sparsa vel in massa gelatinosa.consociata. Cellulse mem- 

 brana loevis vel granulata, achroa vel fuscescens vel rubicunda, saepius longitudinaliter striata. (R.) 



Syn. — Netrium, N^geli. 



Cylindrocystis, Mengh. 

 Glosterium, partim, Ehrenberg. 



Cells cylindrical or fusiform, straight, rounded at each end, or truncately rounded (not emarginate 

 or excised), medianly often constricted. Chlorophyl lamina axillary, when seen transversely radi- 

 ately divergent, arms often forked, and containing starch granules, mostly longitudinally striate. 

 Individuals swimming free in the water, scattered and single, or associated in gelatinous masses. 

 Cell membranes smooth or granulate, transparent or fuscous or reddish, often longitudinally striate. 



a. Lamina chlorophyllacea peripherice lobata vel radiatim expansa. 

 a. Chlorophyl lamina, lobate -on the periphery or radiately expanded. 



P.Disitns, (Ehrb.) Br£b. 



P. cellulis ovato-cylindricis, diametro 3-5 plo longioribus, utroque polo parum attenuatis, sub- 



truncato-rotundatis; laminis chlorophyllaceis peripherice lobatis, medio interruptis. 

 mam.—j\%ji" = .00113"— yf^/ = .0029". 

 • 8yn.— P. Digitus, (Ehrb.) Br^b. Rabenhorst, Flora Europ. Algar., Sect. III. p. 118. 

 Cells ovately cylindrical, 3-5 times as long as broad, at each end slightly attenuate, subtrun, 

 cately rounded ; chlorophyl lamina lobate on the periphery, interrupted in the middle. 



