1888 - 89 .] Mr John Rattray on the Genus Coscinodiscus. 629 
Artificial Key. 
f Circular. Rays confined to a narrow area about 
I semiradius. Central area sharply circum- 
^ 1 scribed. Border narrow, .... multiradiatum. 
' j Angular. Rays passing between central area and 
angles of valve. Central area not sharply cir- 
l cumscribed. Border angular, broad, . , septenarium. 
BRIGHTWELLIA. 
Ralfs, Pritch. Inf., p. 940. — Circular. Surface flat from the 
centre outwards to the circle of large areolae, beyond this slightly 
convex, and sloping downwards to the border. Colour pale 
smoky grey, when dry sometimes purplish or brown. Central 
space circular or obtusely angular, distinct, hyaline ; more rarely 
minute or absent ; a rosette rarely differentiated. Markings areolate, 
rarely subcircular towards the centre; a distinct circlet formed by a 
single row of large areolae, and situated between J and § of radius 
from centre ; within this circlet the rows evident, substraight, or 
oblique, curved, and decussating, beyond the circle radial ; evident 
costae and primary rays sometimes radiating at regular intervals 
between the circlet of large areolae and the border ; minute apiculi 
sometimes present. Border narrow, hyaline, with delicate radial 
striae. — Craspedodiscus, pro parte , Brightw., Quart. Jour. Micr. 
Sci ., 1860, p. 95 ; Heterodictyon, Grev., Trans. Mic. Soc. Lond., 
1863, p. 67. 
This genus approaches Coscinodiscus through C. bidliens. 
§ 1. AcOSTATiE. 
Non-costate. 
B. splendida, Rattray. — Heterodictyon splendidum , Grev., Trans. 
Micr. Soc. Lond., 1863, p. 67, pi. iv. fig. 7. — Diam. -045 mm. 
Central space small, indistinct. Markings toward the centre subcir- 
cular, soon becoming obtusely angular ; the circlet of large areolae at 
about J- of radius from centre, within this circlet the markings about 
4 in '01 mm., in obscure slightly curved radial rows; the secondary 
decussating rows undifferentiated ; the hyaline interspaces largest 
towards the centre ; beyond the circlet the rows moniliform, with 
markings decreasing gradually outwards from 5 to 8 in *01 mm., 
