Vegetative cells cylindrical or constricted at the ends, varying from 1-16 diameters in length; chromatophore varying from an axile plate with two or more pyrenoids to stellate chromatophores, each with a central pyrenoid. Reproduction by zygospores formed of the complete contents of the gametangia; not cut off from the gametangia by partition walls; but in the process of conjugation, as the gametes pass into the conjugating tube, their place is taken by a secretion of cellulose. Aplanospores occupying only part of the sporogenous cell, the remainder being filled with cellulose. All spores variable in form. Parthenospores and akinetes occur not infrequently in some of the species. The walls of the aplanospores and parthenospores resemble the zygospores of the same species in their markings. There are now eleven described species belonging to this genus. _D. immersa_ W. West and _D. africana_ G. S. West bear a close resemblance to _Mongeotia sphaerocarpa_ Wolle. _D. Hardyi_ G. S. West has much the same appearance as _Mongeotia viridis_ (Kutz) Wittrock. _D. desmidiodes_ W. & G. S. West, _D. calospora_ (Palla) W. & G. S. West, _D. reticulata_, _D. americana_, and _D. decussata_ have characters in common with the Zygnemas. _D. glyptosperma_ has the vegetative characters common to several of the species, but its spores are quite unique among the _Zygnemales_. =ZYGNEMA= Agardh. =Z. pectinatum= (Vauch.) Agardh. This is probably common in the eastern half of the United States at least. In Illinois along with the type occurs the variety _conspicuum_ (Hass.) Kirchner, and a variety with large spores. This latter variety in fact is more common than the type. var. =crassum= nov. var. Cellulis vegetativis 30-40µ × 20-80µ; zygosporis 40-55µ × 50-60µ, ceterum ut in typo. Vegetative cells 30-40µ × 20-80µ; zygospores 40-55µ × 50-60µ, otherwise like the type. Type in herb. E. N. T. Collections No. 2350, 2392, 2660, 2685. =Z. ericetorum= (Kütz) Hansgirg. Professor G. S. West has studied the reproduction of this species and finds that it is a true Zygnema and that the description and figure by De Bary, which shows the cutting off of two special gametangia before the union of the gametes is at fault, consequently there is no longer any need of maintaining the genus _Zygogonium_ Kützing. =Z. peliosporum= Wittrock. Specimens of this species have been distributed under the name of _Z. chalybeospernum_ Hansgirg, in P. B.-A. No. 808 from Boswell, Calif. (N. L. Gardner); Amer. Alg. No. 156 from Ft. Collins, Colo. (J. H. Cowan); and Amer. Alg. No. 392 from Vancouver, B. C. (J. E. Tilden). _Z. chalybeospermum_ has the median wall smooth, but the spores of all of the above specimens have distinctly scrobiculate median walls. In size the specimens show a somewhat greater variation in dimensions than has been recorded for European localities. =Z. cruciatum= (Vauch) Agardh.