13 
ON A NEW SPECIES OF DIDYMIUM OCCURRING 
id IN ESSEX. 
BY G, LISTER, F.LS. 
(With one Plate.) 
[Read 25th November, 1922.] 
HE subject of this note was first obtained by Mr. James 
Saunders, A.L.S., in the summer of 1897, when hunting 
for Mycetozoa in prolific heaps of old straw near Barton, Bed- 
fordshire. The small sessile white sporangia resembled those 
of Didymium difforme Duby, with which they were often 
associated, but even in the field they could be generally dis- 
tinguished by the external crust of lime-crystals being not quite 
so smooth and egg-shell-like as in the latter species ; when 
examined microscopically the spores were found to be rough with 
scattered spines, not smooth or marked only with a low branch- 
ing ridge, as in D. difforme. 
In the same straw heap the fragile top-shaped sporangia of 
Didymium vaccinum (Dur. & Mont.) Buchet' were also abundant, 
a species then new to us; so similar are the two forms in some 
respects, it seemed possible that the smaller sessile form might 
be a variety of D. vaccinum. Experience has shown, however, 
that this is not the case ; the sessile form proves to be a widely 
distributed and constant species, retaining its characters in a 
number of different habitats. I propose to name it Didymium 
trachysporum, in reference to the rough spores. It has been 
obtained from the following localities :—from near Luton, Bed- 
fordshire, on old straw, by Mr. Saunders in 1906 ; from Lesmoir, 
Aberdeenshire, on dead grass, by the Rev. W. Cran, in 1913 ; 
repeatedly, in cultures, on old pellets of rabbit and deer, and old 
bean stalks, Berlin, by Dr. Jahn; from near Vienna, also on 
deer-pellets, by Christian Lippert, in about 1895, and from 
Pornic, Loire Inférieure, on old straw, by M. S. Buchet, in 1912. 
In the summer of 1917 it was abundant on old straw manure in 
our garden at Leytonstone, Essex, and also on straw near Theydon 
Bois. During the present summer and autumn the Rev. P. J. 
1 This name replaces Didymium Trochus Lister, M. Samuel Buchet having found that the 
specimen from Algeria, on old Opuntia stem, described as Diderma vaccinum Durieu Montagne 
(published in Exploration Scientifique de V Algerie, 1846, p 407 (22 bis, fig 1 ato h) is the sane 
species; the specific name Tvochus is thus antedated ; M. Buchet has published the com- 
bination Didymium vaccinum in Bulletin de la Société Mycologique de France, xxXxvi., 
Pp. Ifo. (1920), 
H 
