4. Diderma globosum Persoon.

[Plate VII.], Figs. 5, 5 a.

Sporangia more or less closely gregarious, sessile, globose or by mutual pressure prismatic or polyhedral, white, the outer wall smooth, polished, crustaceous, fragile, far remote from the inner, which is thin, smooth, or rugulose, iridescent blue; hypothallus usually pronounced and spreading beyond the sporangia, sometimes scanty or lacking, columella variable, sometimes very small, inconspicuous, sometimes large, globose, ellipsoidal, even pedicellate; capillitium abundant, brown or purplish brown, branching and occasionally anastomosing to form a loosely constructed superficial net; spores globose, delicately spinulose, 8 µ.

This species seems rare in this country. We have specimens from Iowa. It is distinguished by small spores and generally snow-white color. Lister has thrown doubt upon Rostafinski's definition of this form—Mycetozoa, p. 78. Almost everything distributed in the United States under this name belongs in the next species. Reported also from Ohio,—Morgan. Washington. But:—it should be found in Europe, where first described!

There are two ways to meet the difficulty. In the first place it seems probable that a small-spored form really hides somewhere in Europe. The difference between the Monograph measurement and the size admitted for D. crustaceum Pk., evidently considered by Mr. Lister as type and so used in his illustration, Pl. 85, is too great to be esteemed merely an error. That added .3 (Rost.) indicates caution, the average of several measurements. Our D. globosum may represent what the Monograph describes.[32] In the second place we may as American students mistake larger and more globular forms of something else, of D. spumarioides Fr., whose spores are but little larger; or of D. effusum (Schw.) Morg., where the flattened plasmodiocarps anon splatter out to globose drops of polished whiteness, and whose spores are 8 µ. But even here the chances of error are small. In the species last named the columella or sporangial base is alutaceous, not white; in Fries' species, while the columella if present may be white, the peridial walls are different, difficult to distinguish.

For these reasons, D. globosum Pers. may stand, waiting further light from Europe.

5. Diderma crustaceum Peck.

[Plate VII.], Fig. 7