36 Dr Greville’s Descriptions of New Genera 
Hab. New Caledonia ; in a dredging communicated by 
Dr Roberts. Very rare. 
Of this diatom I have only seen two examples; but the 
characters are so remarkable, that there can be no doubt of 
its novelty as a species. The central constriction, it now 
appears, is not uncommon in this genus. 
Plagiogramma constrictum, un. sp., Grev.— Valve con- 
stricted in the middle, then suddenly dilated and lyriform, 
with terminal and central vitte ; strize moniliform, inter- 
rupted, 11 in 001". Length about ‘0025” (fig. 8). 
Hab. New Caledonia; in a dredging communicated by 
Dr Roberts. 
Allied to P. lyratum, but differing in the smaller size of 
the frustule, in the relatively much broader valve, more 
abrupt constriction, more approximated central vitte, and 
much fewer strie. 
Plagiogramma Atomus, n. sp., Grev.— Very minute; valve 
with terminal and central vitte ; the intermediate spaces 
spherically dilated ; striae few, pervious. Length, 0005" 
(fis, 9). 
Hab. Woodlark Island; in a dredging communicated 
by Dr Roberts. 
The smallest of the genus hitherto discovered, distin- 
guished by the very deep constriction and spherical form of 
the dilated portions, each of which contains about half a 
dozen strize. 
OMPHALOPSIS, nov. gen., Grev, 
Frustules united into a filament; lateral view cruciform, 
with central nodule, terminal vitte, and interrupted trans- 
verse strie. } 
This genus is allied on the one hand to Plagiogramma, 
on the other to Glyphodesmis. From the former it differs 
in the absence of central vitte; from the latter, in the 
presence of terminal vitte, as well as in difference of struc- 
ture, the remarkable clathrate cellulation being wholly 
wanting. In the cruciform lateral view it resembles 
Odontidium, (?) Harrisonit of Smith, and O. speciosum of 
Brightwell (both placed by Ralfs in his genus Dimere- 
