since
Persoon's definition in any case seems sufficiently elastic, we have
seen no reason to discard the older name. Persoon's _Diderma_ when
established, _l. c._, included _D. floriforme_. He made some confusion
in his later work by admitting some physarums. This induced Schrader to
throw all the didermas into his new genus, _Didymium_.

According to the nature of the sporangial wall, the species fall rather
naturally into two sections:--

_A._ Outer sporangial wall distinctly calcareous, fragile;
species generally sessile _Diderma_

_B._ Outer sporangial wall cartilaginous, the inner less
distinct, or concrete with the outer; species
oftener stipitate _Leangium_

_A._ Sub-Genus DIDERMA

1. Fructification wholly plasmodiocarpous 1. _D. effusum_

2. Fructification of distinct sporangia.

_a._ Sporangia on a common hypothallus.

O Outer wall fragile, not widely remote from
the inner 2. _D. spumarioides_

OO Inner wall lacking 3. _D. simplex_

OOO Outer wall crustaceous, porcelain-like.

i. Spores 8-10 4. _D. globosum_

ii. Spores 12-15 5. _D. crustaceum_

OOOO Outer wall firm, not crustaceous 6. _D. lyallii_

_b._ Sporangia isolated, or, at least, not on a common hypothallus,
sessile.

O Outer wall porcellanous, roseate 7. _D. testaceum_

OO Outer wall white 8. _D. niveum_

OOO Outer wall ashen 9. _D. cinereum_

_c._ Sporangia stipitate 10. _D. hemisphericum_

_B._ Sub-Genus LEANGIUM

1. Sporangia generally sessile.

_a._ Inner peridium distinct.

O Membranous colorless, columella scant 11. _D. sauteri_

OO Colorless, columella prominent, red 12. _D. cor-rubrum_

OOO Outer ochraceous, inner yellow 13. _D. ochraceum_

_b._ Peridial layers inseparable.

O Peridium multifid; columella small
or none 16. _D. trevelyani_

OO Peridium breaking into but few irregular lobes; columella
prominent.

i. Peridium um