15(j FRESH-WATER ALG^ OF THE UNITED STATES. 



Cells single or geminately concatenate, inflated, profoundly constricted ; semicells compressed, 

 oblong, hemispherical or subqaadrangular, protruding in the centre as a rounded truncate or den- 

 ticulate tubercle. Cytioderm firm, armed with setae, or simple, or bi- tri-furcately divided spines. 

 Chlorophyl radiately expanded. Zygospores armed. 



BemarTe. — It has so happened that I have identified but a single species of this 

 genus. 



X. aculeatum, Ehbb. 



X. parvum, singulum, sparsum, diametro ipse subcequale, ex oblique ellipsoideum, diametro 

 duplo longius, constrictione obtusa lineari, semicellulis oblongis subreniformibus, basi sub- 

 planis, dorso late rotundatis, tuberculo central! minus elevato, truncato, margine autem crenato- 

 dentato ; cytiodermate undiqne acaleis subulatis obsito. (E.) Species mihi ignota. 



Z)mm.— (Sine aculeis) 0.0025"— 0.0029". (R.) 



Syn. — Xanthidium aculeatum, Ehrenbekg. Rabenhorst, Flora Europ. Algarum, Sect. III. 

 p. 222. 



Eab. — Prope Savannah, Greorgia ; Bailey. 



Frond inf. v. broader than long; constriction deep, linear; segments somewhat reniform; 

 spines subulate, short, scattered, chiefly marginal ; central protuberance cylindrical, truncate, 

 border minutely dentate. (A.) 



X. Arctiscon, Ehbb. 



X. semicellulis globosis, binis, aculeatis, aculeis numerosis undique sparsis crassis asperis apice 

 trilobis. (R.) Species mihi ignota. 



Syn.—X. Arctiscon, Ehrenberq. Rabenhorst, Flora Europ. Algarum, Sect. III. p. 224. 

 Hab. — America borealis ; Ehrenberg. 



Frond in f. v. about as long as broad ; constriction forming a wide notch ; segments narrowed 

 at the base, with broadly rounded ends ; spines numerous, restricted to the outer margin, 

 scattered, elongate, stout, terminated by three or four diverging points. (Archer.) 



X. armatum, (Breb.) Ralfs. 



X. maximum, validum, solitarium vel binatim conjunctum, diametro plerumque duplo longius; 

 semicellulis sabcordatis vel angulari-rotundatis tuberculo centrali subelevato, truncato, mar- 

 gine granulato-dentato praeditis ; cytiodermate verruculoso et processibus ssepius geminatis 

 truncatis apice inciso-furcatis instructo. (R.) 



Syn.— Xanthidium armatum, (Brebisson) Ralfs, British Desmidieae et Rabenhorst, Flora 

 Europ. Algarum, Sect. III. p. 222. 



Bffl6.— South Carolina ; Florida ; Bailey. Saco Lake ; (Lewis) Wood. 



Frond large, in f. v. twice as long as broad ; constriction deep, linear ; segments broadest at 

 the base; ends rounded or somewhat truncate; spines in pairs, principally marginal, short, 

 stout, terminated by three or four divergent points ; central projections cylindrical truncate^ 

 the border dentate; e. f. punctate. Sporangium large, orbicular, with depressed tubercles' 

 perhaps immature. L. y|^". B. ^j^". (A.) ' 



Eemark.— Fig. 17, pi. 13, is a front view of a frond, magnified 260 diameters. 



X. bisenarium, Ehrb. 



X. semicellulis globosis subangulosis, binis, aculeatis ; aculeis fasciculatis, fasciculis in quovis 

 globulo senis. Species mihi ignota. 



Syn.—X. bixenarium, Ehrenberg. Rabenhorst, Flora Europ. Algarum, Sect. III. p 224 

 X. Brebissonii, Ralfs. Archer, Pritchard's Infusoria, p. ^36. 



