162 GREVILLE, ON DIATOMACEZ. 
valve transversely to the median ray; rays seven, six of them 
broad linear, terminating at the margin in a narrow, lunate 
fold of the valve. Diameter ‘0018’ to 0070". (Figs. 7, 8.) 
In Casifornian and Peruvian guanos. 
This noble Diatom is of a pale-yellowish colour, somewhat 
variable in outline, but, as a general rule, the transverse 
diameter is the greatest, so that one pair at least of the 
lateral rays are longer than the median ray, the length of 
the latter, as well as the former, being calculated from the 
edge of the hyaline area to the margin of thedisc. The rays 
are uniformly seven in number, and (excluding the median 
ray) of equal width to the very ends, where they terminate 
abruptly under a narrow lunate fold of the valve. The 
median ray, before reaching the margin, passes along a broad 
shallow groove or channel, and terminates in a similar, though 
deeper fold of the valve. The rays, though typically some- 
what arcuate, are occasionally straight; and although their 
excentrical position can always be traced, the fine individual 
I have drawn at fig. 7 will serve to show how nearly centrical 
they sometimes become, and how nearly the valve may 
approach to an orbicular outline. Each ray is bordered 
with a row of areole larger than the rest, especially the 
median ray, where they increase in size towards its extremity. 
The areolation is very distinct, coming out strong and 
hexagonal under a sufficiently magnifying power. 
In certain characters this species comes near to S. heptac- 
tis, viz., in the number of rays, in their being linear and 
subarcuate, and in the “angular sinuosity ” of the lines of 
the hyaline area; a character referred to by De Brébisson, 
although not introduced into his figure. On the other hand 
the form of the valve is the reverse of that of S. heptactis ; 
as I have never seen it otherwise than broadest in the trans- 
verse direction and consequently with the median ray not 
the longest. Fig. 8 represents this typical form. A more 
important differential character lies in the singular termina- 
tion of the rays in a marginal recess or fold of the valve, 
which would scarcely have escaped so acute an observer as 
De Brébisson had it existed in 8. heptactis. It will be seen 
from the figures that the radiating lines of the hyaline area 
are curiously ramified; the main lines being sharp and 
strong, while an obscure branch proceeds from each angle to 
the base of the ray next to it. In bringing these remarks to 
a close, I may state that, although size is of little conse- 
quence in a rigid diagnosis of Diatomacee, S. Ralfsianum is 
a gigantic form as compared with S. heptactis. 
