246 Proceedings of the Royal Irish Academy. 
acceptance as a necessary inference from the fact of the continuous 
existence of numerous species, despite of the law which regulates 
their multiplication by the process of self-division. As in each suc- 
cessive act of fission the newly-formed valves are smaller than those 
within which they haye been secreted, the species would soon become 
extinct, were there not a provision made for its perpetuation in 
the process of sporangial reproduction. All the circumstances consi- 
dered, I am led to regard the Diatomacez as a group of organism on 
which the Creator has impressed certain distinctive characteristics from 
which, through countless, successive ages, they have shown no ten- 
dency to depart. 
LIST: OF SPECIES: 
A. Frustules symmetrical. 1. Valves etreular. 
Famity I. MELOSIREA, Kiitz. 
Frustules simple, or adhering in filaments. Circular on side view. 
Tus family, since the adoption of it by Kiitzing, has undergone 
considerable modification in respect to the genera included within 
it. If we omit the ill-defined genus Pyxidicula, the forms he em- 
braced within it, with the exception of Cyclotella, belonged to those 
genera distinguished by the filamentous character of their growth. 
Kiitzing recognised the analogy between these genera and those of 
which Coscinodiscus may be regarded as the type, but placed them 
widely apart, principally on the ground of the areolate striation of the 
latter. This character, however, is by no means universal, and eyen 
if it were, could scarcely justify so great a dislocation. Grunow, 
therefore, who is followed by Heiberg, includes among the Melosirexe 
all the symmetrical forms circular on the side view, irrespectively of 
their peculiarities of striation; thus establishing a very distinct and 
well-defined group which I adopt—my only difficulty in doing so aris- 
ing from the fact that in the genus Cyclotella, some of the included 
species are waved on the front view, and for this reason can scarcely 
be considered as symmetrical in all aspects, in the sense of Grunow 
and Heiberg. 
Genus I. Merostra, Agardh. 
Frustules filamentous. Convex at the ends, filaments free. 
Melosira borrerti, (Greville.) Marine or brackish water. 
Valves sub-hemispherical; girdlebands marked with conspicuous 
circles of cellules; filaments varying in breadth; colour of the desic- 
cated filaments, a rich brown. (PI. 26, fig. 1.) 
