Sporangia snow-white, clustered, sessile or narrowly adnate, globose or polygonal by mutual compression; peridium double, the outer dense, fragile, thick, calcareous, the inner delicate, remote, translucent, capillitium well developed, the calcareous nodules white, rounded or angular, sometimes uniting to form a pseudo-columella; spore-mass black; spores purplish, distinctly rough, 10–12 µ.

A beautiful and distinct species. As others in the group with which it is here associated, it is a physarum with the outward seeming of a diderma. It occurs in Europe, therefore it is safe to assume that Rostafinski saw it. So well marked it is that any good description will define it, and Rostafinski describes it perfectly, adequately.[23]

Mr. Lister having used for another species the name we here apply—see under P. bitectum—referred this present form to P. didermoides Rost., l. c. Professor Sturgis, convinced that such reference was at least doubtful, gave to our American gatherings the distinctive name above, citing specimens from Massachusetts, from Colorado, and from California. Curiously enough he also includes specimens of R. didermoides var. lividum List., sent from England!

Rare! Certainly rare in Europe and so far seldom seen in the United States, though widely distributed. Specimens are before us from Ohio, Michigan, Iowa, Oregon. No doubt the mountains of the north Pacific coast, a region to-day almost unsearched, will yet afford the species in abundance.

As stated Mr. Lister first applied the name P. diderma to a plasmodiocarpous form occurring in England and near P. sinuosum. More lately, Mon., 2nd ed., p. 78, he adopts a new specific name, P. bitectum for the English specimens, and enters P. diderma as a probable synonym for P. lividum R. Evidently our present form as described above has not come to Mr. Lister's view. He says the original type is not to be consulted.

There is really no more merit in this later comparison than in that discarded. The species P. diderma is not P. lividum, but stands as originally delimited, and will, doubtless, some day yet again appear in its own behalf upon the witness-stand of time; when, as before, a Frenchman in DeBary's old-time haunts may rise to give it welcome, brought back by some keen-eyed Polish student eager now in the arts of peace, from Warsaw's shady groves.

9. Physarum contextum Persoon.

[Plate IX.], Figs. 3 and 3a.