1875. _Didymium clavus_ (Alb. & Schw.) Rost., _Mon._, p. 153.

1899. _Didymium clavus_ (Alb. & Schw.) Rabenh., Macbr., _N. A. S._, p. 90.

1911. _Didymium clavus_ Rost., List., _Mycet., 2nd ed._, p. 128.

Sporangia gregarious, pale gray, discoid or pileate, depressed, stipitate; the peridium dark-colored, frosted with calcareous crystals above, naked below; stipe short, slender, tapering upward, furrowed, arising from a hypothallus more or less distinct, black; columella obsolete; capillitium of delicate threads, pale or colorless, little branched; spores violaceous, pale, nearly smooth, 6-8 .

This species is well differentiated, easy of recognition by reason of its peculiar discoid sporangium, calcareous above, naked and black beneath. _D. neglectum_ Ma.s.see, reported from Philadelphia, is said to be a slender form of the present species. The figures of _D. clavus_ by Albertini and Schweinitz are excellent, as also the description.

Not common. Pennsylvania, Ohio, Iowa.

10. DIDYMIUM NIGRIPES (_Link_) _Fries._

PLATE VII., Figs. 2, 2 _a_, 2 _b_.

1809. _Physarum nigripes_ Link, _Obs. Diss._, I., p. 27.

1818. _Physarum microcarpon_ Fr., _Sym. Gast._, p. 23.

1829. _Didymium nigripes_ (Link) Fr., _Syst. Myc._, III., p. 119.

1875. _Didymium microcarpon_ (Fr.) Rost., _Mon._, p. 157.

1896. _Didymium microcarpon_ Fr., Morg., _Jour. Cin. Soc._, p. 61.

Sporangia gregarious, globose or hemispheric, umbilicate beneath, small, white, stipitate; the peridium smoky, covered with minute calcareous crystals; stipe slender, erect, black, opaque; hypothallus scutate, black; columella distinct, globose, black or dark brown; capillitium of delicate threads, pale brown or colorless, with occasional brown thickenings or nodes, sparingly branched; spores pale, violaceous by transmitted light, minutely warted, 6-8 .

This is _D. microcarpon_ Rost. Fries, _l. c._, acknowledges the priority of Link's appellation, and discards _microcarpon_. Rostafinski adopted _microcarpon_ simply because he thought it more appropriate. Fries describes the columella "none or black." It is doubtful whether we have the typical Friesian form on this continent. The fructification is in our specimens small, about .4 mm., and the spores, as noted by Morgan, small; otherwise the species is hardly more than a variety of the next.

Under the name _D. nigripes_ Lister groups our Nos. 10, 11, 12. _N. A.

F._, 1393, represents Dr. Rex's conception of the present species.

Not common. New York, Ohio, Iowa.

11. DIDYMIUM XANTHOPUS (_Ditmar_) _Fr._

PLATE XVI., Fig. 10.

1817. _Cionium xanthopus_ Ditmar, Sturm, _Deutsch. Fl._, III., p. 37, t. 43.

1829. _Didymium xanthopus_ (Dit.) Fr., _Syst. Myc._, III., p. 120.

1873. _Didymium proximum_ Berk. & C., _Grev._, II., p. 52.

1892. _Didymium microcarpon_ (Fr.) Rost., Macbr., _Bull. Lab. Nat.

Hist. Iowa_, II., p. 146, in part.

1894. _Didymium nigripes_ Fr., List., _Mycetozoa_, p. 98, in part.

Sporangia gregarious, white, globose, slightly umbilicate, stipitate; the peridium thin, and nearly or quite colorless, frosted with crystals of lime; the stipe yellowish or yellowish brown, corneous, erect, subulate, slender; hypothallus none; columella pale or white, turbinate, globose or depressed-globose; capillitium of dull brown, or colorless threads more or less branched, always white at the tips; spores violaceous, nearly smooth, 7.5-8.5 .

This seems to be the most common form in the United States. It is distinguished from the preceding by the longer, more delicate, generally orange-yellow, stem with pale or white columella. The spores also average a shade larger. _N. A. F._, 412 and 2089, are ill.u.s.trations of _D. xanthopus_. The columella in blown-out specimens is very striking, well confirming the diagnosis of Fries, "_valde prominens, globosa, stipitata, alba_." Berkeley makes the color of the capillitium diagnostic of _D. proximum_, but this feature is insufficient.

Eastern United States; common.

12. DIDYMIUM EXIMIUM _Peck._

PLATE XVI., Figs. 11, 11 _a_, 11 _b_.

1879. _Didymium eximium_ Peck, _Rep. N. Y. Mus._, x.x.xI., p. 41.

Sporangia scattered, dull grayish-yellow or gray, depressed-globose, umbilicate, minute, stipitate; the peridium comparatively thick, tenacious, especially persistent below, tawny or yellow; the stipe pale brown or orange, erect, even or slightly enlarged at base; hypothallus scant or none; columella prominent, more or less discoidal, rough, or spinulose, especially on the upper surface, yellow; capillitium not abundant, pale fuliginous, often branching and anastomosing so as to form a loose net; spores nearly smooth, dark violaceous by transmitted light, 8.5-9.5 .

The species differs from _D. xanthopus_ in several particulars,--in the much firmer, more persistent, and less calcareous peridium, in the more complex capillitium, in the darker and larger spores, and especially in the peculiar and prominent columella, which is not only rough, but even "sometimes spinulose even to the extent of long spicules penetrating to one-third the height of the sporangia." _N. A. F._, 2493.

As stated under No. 8, these last two species are called varieties only of _D. nigripes_. They are so retained in _Mycetozoa, 2nd ed._ Since, however, they are the usual presentation of the species in the United States, it seems wise to let them stand for the present, as here. They are quite distinguishable; _D. eximium_ especially well marked.

Apparently rare, it yet ranges from New York to eastern Iowa, in colonies rather large. Okoboji Lake;--fine!

13. DIDYMIUM TROCHUS _List._

1898. _Didymium trochus_ List., _Jour. Bot._, x.x.xVI., p. 164.

Sporangia plasmodiocarpous, hemispherical or turbinate, white, sessile or very short-stalked, cream-colored or white; peridium double, the outer sh.e.l.l-like, the inner membranaceous, more or less adherent to the outer, both caducous together, leaving the thickened base surrounding an expanded columella; stipe, when present, very short, stout; capillitium colorless, nearly simple; spores brownish-purple, strongly warted, 9-10 .

On decaying leaves, rotten cactus, yucca, etc., Monrovia, California; _Bethel_.

Reported from England on beds of leaves or straw; in Portugal Dr.

Torrend finds it on or _in_ dead leaves of _Agave americana_! Evidently an American species, and belonging to arid regions; its occurrence in England surprising!

14. DIDYMIUM ANNULATUM _Macbr. n. s._

PLATE XX., Figs. 4, 4 _a_.

Sporangia small, scattered, annulate, not only without columella but perforate when the stipe is broken, umbilicate above and below, grey, coated with crystalline frustules, opening irregularly about the periphery; stipe white, or pallid, fluted, tapering upward from a distinct hypothallus; capillitium scanty consisting of delicate, spa.r.s.ely branching threads, the branchlets anastomosing more or less at length, attached to the peridial wall, radiating from the rim of the slightly depressed top of stipe, without special thickenings save at the insertion of the ramules a triangular enlargement is usual and of dark or pallid shade; spores smooth; however they show three or four spots on the hemisphere and other minute but variable markings; 9-10 .

Seattle, Washington.

Differs from _D. nigripes_ in color of the stipes, capillitium, spore-diameter, etc.

15. DIDYMIUM DUBIUM _Rost._

1875. _Didymium dubium_ Rost., _Mon._, p. 152.

1892. _Didymium listeri_ Ma.s.s., _Mon._, p. 244.

1894. _Didymium dubium_ Rost., List., _Mycetozoa_, p. 95.

1911. _Didymium dubium_ Rost., List., _Mycetozoa, 2nd ed._, p. 126.

Fructification wholly plasmodiocarpous, snow-white, small, 2-6 mm., flat and thin; the outer wall double, membranous within, calcareous-crystalline without; columella none; capillitium simple of rather thick, vertical, brown threads, sparingly united laterally, and only occasionally furcate at the ends, especially above; spores minutely spinulescent, violaceous pale, 12-15 .

Ma.s.see thought English specimens out of harmony with the original description and gave them a new name. To refuse this, Lister enlarges the range of spore-measurements and disregards some of Rostafinski's specifications as to capillitium. Our specimens are as described.

Bohemia. England. Sh.o.r.es of Lake Okoboji, Iowa.

This is indeed a doubtful form. It differs from _D. difforme_ chiefly in that the outer calcareous sh.e.l.l is not smooth, but is covered with abundant loose crystals, frosted. The spores are paler but about the same size. The frosting may be incident to local climatic conditions at the time and place of desiccation.