TRANSACTIONS. 
Descriptions o/New and Rare Diatoms. Series XIV. 
By R. K. Grevills, LL.D., F.R.S.E., &c. 
■ (Read Nov. 9tli, 1864.) 
(Plates I & II.) 
Plagiogramma. 
Plagiogramma WallicManum, n. sp._, Grev. — Valve linear^ 
rounded at the ends ; costie two in the middle^ and one 
each end^ with a few intermediate pervious striae. (Figs. 
7, 8.) 
Hab, St. Helena ; rare ; Dr. Wallicli. 
A minute^ but well-marked species, of which I find a cha- 
racteristic sketch of the valve in Dr. Wallich^s note-book. 
I have also obtained views of the frustule in both aspects^ 
in the portion of the dredging he was kind enough to place 
in my hands. It is not very closely allied to any described 
species, differing materially from P. pygmcEum, to which it 
approaches in size, in the pervious striae, and strictly linear 
form. Length '001 
Pyxilla, n. gen.^ Grev. 
Frustules free, oblong, transversely bivalved, box-like, 
minutely cellulate ; each valve terminating in a short, thick 
apiculus. 
This genus must obviously be associated with the Pyxidi- 
cul(2 ; indeed, the two species of which it is composed, 
might almost have been placed in Pyxidicula itself, so loosely 
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