418 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Ceratopoka diciiotoma, sp. nov. PI. 2, fig. 11-12. PI. 3, figs 
14-16. PI. 4, figs. 1-18. 
Description . Corallum free above the base, prostrate, lower sur¬ 
face of tubes flattened, upper surface rounded or abruptly sloping, 
with a carination along the middle. Cross-section of the lower por¬ 
tion of the tube subtriangular. Tubes gently and regularly increas¬ 
ing in diameter towards the aperture, before reaching which, in 
adult individuals, they are abruptly bent upwards, so that the calyx 
opens at right angles to the longitudinal axis of the tube. Calyx 
frequently deep, circular in cross-section. 
New tubes given off in pairs, from the baso-lateral angles, diver¬ 
ging at right angles, and each in turn giving off a pair of tubes. 
Seen on the flattened side, the corallites appear to divide dichoto- 
mously. Surface formed by a thick, wrinkled epitheca. Cysts 
moderately coarse, best developed in the larger specimens. The 
cysts frequently change the triangular inner section of the procum¬ 
bent portion of the tube to nearly circular, by cutting off the carina¬ 
tion and the lateral angles. 
Trabeculae numerous, arranged in regular rows; most prominent 
on the walls of the calyx. In the procumbent portion of the tube 
the rows of trabeculae gradually diverge forwards, new ones coming 
in between the older rows. The connecting pore between corallites 
is usually thickly studded with trabeculae. 
This species might at first be taken for an Aulopora, but is readily 
distinguished by its free, though prostrate habit, its triangular cross- 
section, the carination along the center, the abruptly upward bending 
calyx, and the regular dichotomous branching. Internally the cysts 
and trabeculae form distinctive characters. From C. jacksoni and 
C. clistorta it can be distinguished by its regular form and manner 
of branching, and by the carination along the middle of the upper 
side. 
Ontogeny. Only the later stages in the development of the coral - 
lum have been observed. These later stages have not been found 
on an initial corallite, except as indicated by the lines of growth, 
but they can be seen in the secondary corallites. Inasmuch, how¬ 
ever, as each corallite repeats the history of the initial one, excepting 
the earliest stages, we may gain a knowledge of the development of 
the whole corallum — with the exception of the earliest stages — 
by the study of the last formed corallites. 
