44 ON THE GENERA ANADYOMENE AND MICRODICTYON. 
venation offers some peculiarities which perhaps may lead to its specific 
separation. In our Key-West plants the seriated cells of the principal 
veins stand apart from each other, or are in single file, having wedge- 
shaped spaces between. In Mr. Menzies’ specimen the principal veins 
are partly unicellular, partly formed of several parallel closely-placed 
cells without interspaces; the structure is easily seen, but difficult to 
escribe in intelligible lariguage. Should subsequent observation 
establish this plant as a species, it may be called 4. Menziesii-” 
(Harvey, Nereis Boreali-A mericana, iii. 50.) I did not discover this 
observation until after I had described the genus; and I may observe 
that the simple series of cells is only found, in the larger specimens in 
the British Museum, in one or two of the smaller lateral branches near ^ 
the circumference of the frond; all the others are formed of fan-shaped 
series: of cells, from. three to five being in each cross-series, and I am 
more confirmed in this opinion, as I believe there are more than one 
species of the same form with the typical 4nadyomene from very different 
localities, which may be characterized by the form of the cells, and all 
these species agree in having the main stem formed of a single series 
of cells very unlike the many-cellular midribs of Mr. Menzies’ species 
from Mexico. 
It is to be observed that Montagne, when he first observed the 
Microdictyon, called it a second species of Anadyomene, and the cha- 
racter that he gave to distinguish the species was used by Decaisne to 
separate the two genera, and it is quoted by Kiitzing as the specific 
character ofthe species of Microdictyon, although it was drawn up to 
distinguish it from 4. stellata. 
I may perhaps be regarded as unwise in forming a genus of a plant 
that Professor Harvey regards even as a doubtful species. I have 
not done so without great consideration; but when I know that there 
are at least four, if not more specimens of Mr. Menzies’ Mexican plant 
in collections, viz. the one in the British Museum, one at Kew, one.in 
Dr. Harvey’s collection at Trinity College, Dublin, and one or more in 
Mr. Menzies’ own collection, which he left to the Edinburgh Botanic 
Garden, I cannot but regard it as a distinct form ; indeed, Professor 
Harvey, in a note lately received from him, admits its being so. 
Now, if it is a distinct plant,as it presents a very different organization 
tothe other species, which it undoubtedly does, surely that is enough to 
.. form it into a genus. I believe that it is a genus likely to meet with 
