The genus is marked by its surface-net supported at the tips of the dichotomously branched divisions of the columella. Over the net is spread, theoretically at least, the peridial film supported by very short points projecting from the net,--the peridial processes; the peridium, however, is seldom seen; in some cases, certainly, is never developed. Rostafinski first defined the genus as employed by recent writers. Gleditsch simply renamed Micheli's _Clathroidastrum_; all writers subsequent included species of other genera. The taxonomy of this genus is of the most difficult. Macroscopic, defining characters are few, and even these sometimes uncertain. Microscopic distinctions also tend to be illusive, variable in such fashion that often at the critical point the most exact description fails. All that may be done at present is to recognize two or three definite types and then cautiously differentiate among these with the light we have, until more general study of the group brings to service a wider range of observation with more comprehensive record on which judgment may better be sustained. We have before us many and beautiful forms of this genus yet unstudied. Some of these doubtless have already found place in our growing taxonomic literature; some apparently undescribed; all to wait wider leisure or perhaps a younger hand. The entire life-history of every form is none too much if we would set out with any hope of accuracy the genetic relationships for which taxonomy stands. Recently European students are making the color of the plasmodium a basis for species-discrimination, which is good so far. But plasmodic characters are at present unserviceable generally, for two reasons; they vary in the same species; and unfortunately, when most needed, they are unknown and inaccessible. The student is generally confronted by forms mature, the plasmodic stage already past. =Key to the Species of Stemonitis= _A._ Sporangia connately united. _a._ Spores verruculose 1. _S. confluens_ _b._ Spores reticulate 2. _S. trechispora_ _B._ Sporangia at maturity distinct. _a._ Spore-mass grayish black. 1. Larger, 8-12 mm. spores distinctly reticulate or warted, but sometimes nearly smooth 3. _S. fusca_ 2. Spores reticulate and spinulose. i. Spores adherent, clustered 4. _S. uvifera_ ii. Sporangia very tall, 15-20 mm., rigid 5. _S. dictyospora_ iii. Sporangia short, jet- or violet-black 6. _S. nigrescens_ _b._ Spore-mass rich brown. 1. Columella central. i. Sporangia shorter, 5-6 mm., spores banded 7. _S. virginiensis_