82 GREVILLE, ON DIATOMACEA. 
resemble each other. On closer observation, however, cer- 
tain differences do constantly present themselves. In both, 
the most strikmg character is the beautiful radiation, which 
Professor Gregory aptly compared to that exhibited in the 
stars of some orders of knighthood. The lines thus formed 
by the areolation are grouped into what, in the absence of a 
better term, may be called fasciculi, and they appear to con- 
stitute a good discriminative character. In the Hull disc, 
these fasciculi, as they become well defined in their approach 
towards the margin, are composed of about six rows of 
areolz ; in the American disc, on the contrary, the fasciculi 
contain double the number of rows, and are themselves, con- 
sequently, proportionably less numerous. The line of separa- 
tion between the fasciculi is also more evident, causing the 
appearance, under a low power, of a slight undulation of the 
surface, which effect is heightened by the straight oblique 
rows of the more regular areole. If the margin be brought 
into focus, the superior regularity of the areolation becomes still 
more apparent in the series of little intersecting arches which 
spring, as it were, from the inner edge of the border. These 
differences, not very great perhaps in themselves, yet arising, 
as they must do, from the structure of the valve, incline me 
to regard the two forms as distinct. 
With regard to the disc found on the “ Dutch rushes,” I 
cannot speak with certainty. No doubt the general character 
of the areolation (including the radiating fasciculi) is very 
similar to that of C. Normanni. But Mr. Norman has 
pointed out to me the fact (which I have also since observed 
myself) that frustules in a state of union present, on the 
front view, a decided undulation, indicating, in this respect, 
some affinity with Cyclotella punctata. Sm. Syn. V, 2, p. 87, 
a fresh-water species. 
In the centre of the dise of C. Normanni may be gene- 
rally perceived a small irregular, interrupted, opaque circle ; 
as if there were either minute prominences of some extra- 
neous matter, or as if some of the areole were filled with it. 
But this appearance is not invariably present. It is most 
conspicuous under a low magnifying power. 
Having found it difficult to convey a correct idea of the 
structure by means of a figure on the usual scale, I have 
enlarged the one given in the plate x 800 diameters. 
4. Nitzschia arcuata, Greg. MSS. 
Front view of frustule linear, arcuate, rounded at the 
ends; side view lanceolate, obtuse ; length ‘0038’; puncta, 
about 20 im ‘001. (Figs, 4—-7.) 
