GREVILLE, ON PLAGIOGRAMMA, 211 
value in the diagnosis, but adding to the singular aspect of 
the frustule. It seems to be ver y rare in both stations. 
8. P. pygmeum, n. sp., Grev.— Minute ; valve narrow-ob- 
long; length -0012"; strie moniliform, 21 in -001”. (Fig. 
11. y 
Nassau, New Providence, in scrapings of conch shells. 
This diatom is distinguished among its congeners for its 
minute size, its shape, and the small number of striz ; 
for although they are, in the relative proportion assigned 
above, there are, in fact, but few in the entire frustule. 
The valve is not unlike the lower valves of some small 
Achnanthes, but the decided terminal vitte serve at once 
to indicate other affinities. 
9. P. obesum, n. sp., Grev.—Minute; valve broadly di- 
lated in the middle, the ends rounded; length -0022”, 
breadth -0009"; strize 11 im -001". (Figs. 12, 13.) 
In scrapings of conch shells, Nassau, New Providence. 
The inflated appearance of the valve and the small number 
of strize render this a well-marked species. 
10. P. lyratum, n. sp., Grev.—Valve contracted in the 
middle, then dilated and narrowly lyriform, linear and 
rounded at the extremities; length 0042”; strie 18 in ‘001”. 
(Fig. 14.) 
In scrapings of conch shells, Nassau, New Providence. 
The variety of contour exhibited in the valve of this and 
_ the preceding species is a striking illustration of the neces- 
sity of obtaining a side view of the frustule in order to 
make up a complete character, although a sufficient pro- 
visional character may sometimes be furnished by the front 
view. ‘The present diatom, the most beautiful and remark- 
able of its genus, seems very rare, as I have only met with 
one example. 
Section I1I.—Number of vittze between the two centrical 
ones and the ends of the valve indefinite. 
ll. P. californicum, un. sp., Grev.—Valve linear, rounded at 
the ends; vittze between the centre and apices 3—5 ; length 
"0030" to -0038"; strie 18 im 001”. (Figs. 15—17.) 
In Californian guano. 
The only species which has come under my notice belong- 
ing to this section. 
