65 
Dr Grevilie on two new Genera of Byssoideai. 
It is extremely difficult to form a character for this genus, 
though it is remarkably distinct in its general appearance. 
M. ferrugineum I have had in my possession upwards of two 
years, but hesitated to constitute a genus to receive it, until I re- 
ceived another plant from my friend Sir William Jardine, ob- 
viously belonging to the same genus. 
From Sporotrichum , Macrotrichum differs in having the 
filaments suberect, much more robust and rigid, as well as 
much shorter ; the summits of the branches are also obtuse, and 
they are nearly of an equal diameter throughout Besides, in 
M. ferrugineum the sporidia are evidently at one period at- 
tached to the filaments. M. heterosporum has a remote resem- 
blance to Oidium ; but in that genus the joints of the flocci se- 
parate spontaneously into oval pseudo-sporidia , besides which, 
the flocci themselves are lax and entangled. 
PL I. Fig. 1. Plant natural size. 2. Filaments and sporidia 
magnified. 
Eurotium Rosarum, Grev. 
E. caspitoso-ejfusum , sericeum ; peridiis gr eg arils, vir ides - 
centibus, Jlocis tectis ; floccis elongatis , confertissimis , sim- 
plicibus , medio ascendentibus. 
Hab. In caulibus, ramis, et capsulis adhuc viridibus Rosa- 
rum ; aestate et autumno. 
Desc. Flocci septati, effusi, repentes, ramos demum amplex- 
antes, densissime csespitosi, medium versus erecti, confer- 
tissimi, sericei, albidi, demum pallido-rufescentes, ramis cap- 
sulisque arete adhosrentes. Peridia minuta, numerosa, 
viridi-fuscescentia, primo subconspicua, demum floccibus 
tecta. Spondee minutissimse, globosas, rariores. 
This plant has at first sight but little of the habit of the ge- 
nus in which I have placed it ; or, rather, it does not resemble 
any of the species of Eurotium already known. 
Under the miscroscope, however, the generic character is suf- 
ficiently evident, and the structure of the peridium satisfactory. 
The long byssoid filaments are obviously attached to the base 
of the peridia, and seem to arise from it exactly as in the genus 
Erysiphe ( Alphitomorpha , Wallr.), from which Eurotium only 
VOL. XIII. NO. 25. JULY 1825. 
E 
