167 
with descriptions of new genera and species. 
tioii ; and it is interesting to find that differences are observable 
in the character and position of the sporangia sufficient to justify 
a removal of some of the species from the genus Meloseira, in 
which they are at present included by Professor Kiitzing in his 
valuable work on the Diatomacece. A careful examination shows 
toOj that, independently of the difference in the sporangia, there 
is sufficient distinction between the frustules themselves to cha- 
racterize the proposed new genera. 
Although it is very probable that hereafter it will be found 
desirable to break up still further the genus Meloseira, it is pro- 
posed at present only to separate from it — 1st. Those species 
characterized by the absence in the frustule of an evident cen- 
tral line indicating the place of subsequent fissiparous division, 
but each frustule having two somewhat distant sulci or fossulse 
passing round it — Aulacoseira. 2ndly. Those species, the frus- 
tules of which are not at all convex at the extremities, and which 
therefore form by their close contact an uninterrupted cylindrical 
filament ; each frustule is marked with a central line and its in- 
ternal cavity is spherical or subspherical — Orthoseira, 
These two new genera may be defined as follows : — 
Aulxcoseira. — Cellulis cylindricis bisulcatis extremitatibus plus 
minusve rotundatis in filamenta concatenatis. 
Typ. spec. Meloseira crenulata, Kutz. = Af. orichalcea, Ralfs. 
Orthoseira. — Cellulis exacte cylindricis linea centrali notatis 
in filamenta cylindrica connexis ; cavitatibus internis sphsericis 
vel subsphsericis. 
Typ. spec. Meloseira aniericana, Kiitz. 
The genus Meloseira, as it stands after this removal of some 
of its species, will include all those whose frustules are in any 
degree convex at their extremities, and have the central line in- 
dicating the place of future fissiparous division. It will probably 
be found expedient to separate Meloseira arenaria, Moore, from 
its present congeners when its sporangia have been discovered. 
Sporangia or sporangial frustules have been observed by the 
writer in the following species of the genus Meloseira as now re- 
stricted, viz. in M. varians, Ag., M. nummuloides, Ag., M. Bor- 
reri, Grev., and in an Antarctic species collected by Dr. Hooker 
allied to M. globifera, lialfs. In these species the sporangium is 
spherical with its axis of growth corresponding with that of the 
filament in which it is situated, and to which it continues for some 
time closely to adhere. The sporangium of M. varians, Ag., has 
one, sometimes two projections or mammillse, each of which fits 
into an empty half- frustule, and frequently so closely as to be 
inseparable from it. PI. XI. fig. A I represents various forms 
and stages of development of the sporangium of M. varians] and 
