TRANSACTIONS. 
Descriptions of New and Eare Diatoms. Series XVIII. 
By R. K. Greville, LL.D., F.R.S.E., &c. 
(Coraraunicated by P. C. S. Ropeu, F.L.S., &c.) 
(Read Nov. 8th, 1865.) 
(Plates I & II.) 
Plagiogramma. 
Plagiogramma decussatum, n. sp._, Grev. — Valve elliptic- 
oblong, with 2 central costse and numerous pervious striae 
composed of minute granules,, so arranged as to form decus- 
sating lines. Length '0022'^ (Pigs. 1—2.) 
Hab. St. Helena^ in fifteen fathoms ; Dr. Wallich. Shark''s 
Bay, west coast of Australia, in stomachs of Ascidians ; Dr, 
Macdonald. Zanzibar; Professor Hamilton Smith. 
This species is so exceedingly like P. Gregorianum [Den- 
ticula Staurophora, Greg.) that it requires careful examination 
to detect the difierence. One character, however, is amply 
sufficient to separate them. In P. Gregorianum the pervious 
strise are merely obscurely moniliform, whereas in the 
species under consideration, under the same magnifying 
power, they are seen to be composed of distinct, somewhat 
transversely oblong granules, so regularly arranged that they 
form distinct longitudinal and transverse decussating lines. 
The valve is also considerably more robust than that of P» 
Gregorianum. The discovery of this species is due to Dr. 
Wallich, in whose notes and sketches it is clearly indicated. 
Plagiogramma Barbadense, n. sp., Grev. — Valve narrow, 
elongated, contracted in the middle, then dilated, and 
again contracted into linear subacute extremities ; costse 2, 
strong, centrical; structure showing exceedingly fine longi- 
tudinal and transverse lines (dots) and another series of 
numerous very fine transverse pervious strise. Length 
•0035". (Fig. 3.) 
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