GREVILLE, ON PLAGIOGRAMMA. 209 
strie interrupted, composed of large subquadrate granules, 
8 in ‘001"; length of frustule 0040”. (Fig. 7.) 
In Californian guano. 
A single example of this most remarkable diatom is all 
that I have seen; and I introduce it with considerable hesi- 
tation, for I am by no means satisfied that it belongs to the 
present genus. 
Section II.—Vittz 2, centrical, and one at each end of the 
valve. 
4. P. pulchellum, un. sp., Grev.—Valve linear-elliptical, 
obtuse ; striz interrupted, robust, conspicuously moniliform, 
11 in -001”; length of frustule -0025"” to -0057”. (Figs. 
4—6.) 
In Californian guano; Jamaica, in washings of alge ; 
Nassau, New Providence, in scrapings of conch shells. 
Of this fine species I have seen as many as five frustules 
united in a chain, but this is extremely rare; for their cohe- 
rence in the whole genus seems to be very slight. The 
terminal vitte are rather distant from the extremities of the 
valve, and two or three faint and short striz are seen on the 
side of the vitta next the apex. The striz are very strong, 
and conspicuously moniliform. It will be perceived how 
greatly this species varies with regard to size; but I can- 
not detect any real difference between the large frustule 
represented at fig. 6 and the smaller ones forming the chain 
at fig. 5. There is another form, of which I have only seen 
the front view, which differs in the striz being shorter and 
slightly more numerous (13 in ‘001"). It occurs in Califor- 
nian guano and in the West Indies. 
5. P. validum, un. sp., Grev.—Valve linear, slightly di- 
lated in the middle, rounded at the ends; striz interrupted, 
conspicuously moniliform, 12 in ‘001"; length of frustule 
"0055", breadth -0007”. (Fig. 8.) 
In Californian guano. 
As in the preceding species, I have only seen the side view 
of this diatom, the character of which is abundantly dis- 
tinct. The valve maintains its breadth to the extremities. 
6. P. ornatum, u. sp., Grev.—Valve ...? striz in front 
view of frustule broad, moniliform-costate, 8 or 9 in ‘001"; 
connecting membrane with longitudinal rows of dots 15 in 
001"; length of frustule -0052". (Fig. 9.) 
In Californian guano. 
