207 
ORIGINAL COMMUNICATIONS. 
On PLacioGRAMMA, @ NEW Genus of DiaTomacrEa. 
By R. K. Grevitxiz, LL.D., F.R.S.E., &e. 
Tur only known species of the new genus of Diatomacez, 
which I propose to establish in this communication, was 
published by my late friend, Professor Gregory, in 1857, in 
his paper “On the Marine Diatomacez of the Clyde”’ (‘ Trans. 
Roy. Soe. Ed.,’ vol. xxi), under the name of Denticula stau- 
rophora. Soon afterwards, two or three other evidently 
nearly allied diatoms were observed by Professor Walker- 
Arnott and myself in Californian guano; and my attention 
having been thus attracted to these remarkable forms, I lost 
no subsequent opportunity of searching for them, both in 
guano preparations and recent gatherings. Some I have de- 
tected in a gathering obtained by washing small alge picked 
up on the coast of Jamaica; and several very interesting 
species occurred in the scrapings of conch shells from Nassau, 
New Providence, kindly sent to me by Mr. Norman, of Hull. 
The Caribbean Sea appears to be particularly rich in Diato-. 
macee; and there can be no doubt that this curious little 
group will be further enlarged when observers shall have been 
supplied with additional materials from its shores.* 
With regard to the affinities of this genus, it seems to be 
most nearly allied to Odontidiwm and Denticula; but the 
presence of vittze constitutes an important difference. In the 
place of costz, also, the species are furnished in almost every 
instance with conspicuous moniliform striz. There is, how- 
ever, in P. ornata an approach towards the coste of Odonii- 
dium. The striz are generally interrupted. The vitte are 
composed sometimes of a centrical pair; sometimes with the 
addition of a solitary one at each extremity of the valve; in 
one case of a centrical pair, with an indefinite additional 
number. 
* T take the opportunity of recommending that those who are interested 
in this subject, and have friends in the West Indies, should endeavour to 
prevail upon them to collect the smaller sea-weeds, especially such as are 
cast ashore in quiet bays and creeks. It should be explained to them that 
the ubject is not the sea-weeds themselves, but the diatoms to be obtained 
from them, and that those sea-weeds are to be preferred which are covered 
VOL. VII. 8 
