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15RIGHTWELUA.
Sect. II. Brightwellie.e. Valves cellular, with one or
more rows of marginal or sub-marginal cells, differing
(generally much larger) ;
from the other cells of the valve or
with the sculpture of the median portion different from that of
the margins.
GENUS 177. BRIGHTWELLIA RALFS., 1861.
Fig. 266, Brighhvellia hyperbvrea.
Valves circular, with cellular structure ;
cells unequal, those of the margin smaller
than those of the centre, and separated
from one another by a ring of much
larger cells.
This beautiful genus possesses seven
species, which are all fossil, two of them
(including B. hyperborea Gritn.), (fig. 266),
have also been found in marine dredgings or
soundings.
GENUS 178. HETERODICTYON GREV., 1863.
Fig. 267. Ilcterodictyon Rylandsianum.
Valves circular, finely punctate,
with a central rosette of medium
sized cells, and a ring, quite margi-
nal, of large elongated cells.
This genus consists of a single
species, H. Rylandsianum Grev.,
figured in the text, found in the Cam-
bridge Estate Deposit, Barbadoes, by
C. Johnson.