158 FRESH -WATER A L G M OF TUE UNITED STATLS. 



Remarlcs. I have found only a single undescribcd species of this genus, but the 

 following European forms have been detected in this country by Prof. Bailey. The 

 genus appears to be, as Prof. Rabenhorst says, scarcely distinguishable from Xa- 

 thidium or Staarastrum. 



A. octocornis, EURB. 



A. parvns, Isevis, constrictione lata excavata; semicellulis trapezoideis, inciso-quadriradiatis, 

 radiis in aculeum acutissiinum strictum porreetis, a latcre clongato-ellipticis, diaruetro fere 

 triplo longiorilms, utroquc polo aculeum singulum gercntibus. (R.) 



Diam. 0.00065". (R.) 



Syn. Xanthidium octocorne, RALFS. BAILEY, Microscopical Observations, p. 29. 



Arthrodesmus octocornis, EURENBERG. RABENHORST, Flora Europ. Algarum, Sect. III. 

 p. 223. 



Hab. Florida ; Rhode Island ; Bailey. 



Frond smooth, minute, about as long as broad ; constriction a wide notch ; segments much 

 compressed, trapezoid, each angle terminated by one or two straight, subulate, acute spines, 

 the intervals between the angles concave. (A.) 



a. Spine solitary at each angle. L. T3 Vr"- B. TjVs"- (-A-) 



b. Larger spines geminate at each angle. L. -roVs"- B. $iY'. (A) 



A. quadridcns, WOOD. 



A. late ovalis, vel suborbicularis, paulum longior quam latus, cum margine crenato-undulato; 

 semicellulis nonnihil reniformibus, ntroque fine aculeo subulate, modice robusto, acuto, re- 

 curvo, armatis ; cytiodermate cum verruculis paucibus modice minutis in seriebus paucibus 

 dispositis instructo; semicellulis a vertice acute ellipticis, et cum margine crenato ct super- 

 ficie sparse verruculosa. 



Diam. Lat. ^^" = .00075" ; long. ?5 V' = .00125". 

 Syn. A. quadridens, Wood, Proc. A. N. S. 180 9. 

 Hab. In lacu Saco, (Lewis) Wood. 



Broadly oval or suborbicular, a little longer than broad, with the margin crenately undulate ; 

 semicells somewhat reniform, at each end armed with a subulate, moderately robust, acute, 

 recurved large spine ; cytioderm with a few smallish tubercles arranged in three or four rows ; 

 semicells from the vertex acutely elliptical, with the margin crenate and the surface sparsely 

 warty. 



Remarks. This species approximates A. divergens, from which it differs in the 

 arrangement of its granules, its attaining not one-half the size, and, I believe, in 

 the larger and more robust spines. 



Fig. 2, pi. 20, represents an empty frond of this species. 



A. Incus, (BREB.) HASSAL. 



A. parvus tarn longus quam latus, constrictione linear! obtusa vel late excisa; semicellulis 

 oblongo-quadrangularibus, angulis externis aculeatis, internis rotundatis inermibus, aculeis 

 longis singulis divergentibus. (R.) 



Diam. Max. 0.00098". Long. 0.00091". Spor. (sine acul.) 0.00085". 



Syn. A. Incus, (BREBISSON) HASSAL, Fresh-Water Algse, p. 357, et RABENHORST, Flora 



Europ. Algarum, Sect. III. p. 226. 

 flab Georgia; Florida; South Carolina ; Rhode Island ; Bailey. 



Frond minute, smooth, as long, or longer than broad, constrictions a deep notch or sinus; seg- 

 ments with inner margin turgid, outer truncate; spines subulate, acute ; sporangium orbicular, 

 spinous; spines subulate. (Archyr) 



