1888-89.] Mr John Rattray on the Genus Coscinodiscus. 677 
pale grey, the processes subhyaline. Markings punctiform or 
areolate, forming evident radial rows or striae; radial costae at 
subresular intervals, sometimes distinct. Process single, springing 
from centre of valve, proximal portion nodular or elongated and 
columnar, distal portion biramose, the rami equal, diverging sym- 
metrically, their outer ends swollen and knob-like, rounded or 
elliptical. 
This genus forms the transition between the circular forms of 
the Diatomaceae and the armed Chadocerotidce. Apart from the 
process, the valves approach Coscinodiscus , and the nodular 
proximal portion of that of Thaumatonema costatum is but a 
greater development of, and so homologous with, the nodule of 
Coscinodiscus nodulifer , this development being still more marked 
in T. barbadense. 
T. barbadense. Grev., Trans. Micr. Soc. Lond., 1863, p. 76, 
pi. v. fig. 26. — Circular. Diam. *03 to *04 mm.; height of central 
portion of valve above edge of girdle ’075 to T mm. Surface flat 
to about § of radius. Markings punctiform, closely arranged in 
evident striae, the striae 6 to 6| in ’01 mm. Process ’0185 to 
•0315 mm. long, with proximal portion columnar, the rami of the 
upper portion stout, distance between the extremities of the rami 
•0135 to ’0185 mm. 
Habitat. — Cambridge deposit, Barbados (Johnson !); “ Barbados ” 
(Johnson !). 
T. costatum. Grev., Trans . Micr. Soc. Lond., 1865, p. 97, 
pi. viii. fig. 3. — Fragmentary. Roundly elliptical. Major axis 
’055 mm., about 1^ times minor. Surface flat to about semiradius. 
Markings areolate, increasing slightly to about semiradius, thence 
decreasing gradually and becoming more faint to border ; at semi- 
radius 5, at border 8 to 9, in ’01 mm., at subregular intervals of 
about ’0125 mm., radiating evident costae. Process evident, the 
proximal median portion nodular, the diverging rami more delicate, 
straight, their outer ends elliptical, knob-like ; length of rami 
including terminal knob ’0175 mm., major axis of knob ’01 mm., 
about 2J times minor. 
Habitat. — Cambridge deposit, Barbados (Johnson !). 
