236 
the corallum there are a great number of smaller calicles. In 
Dana’s figure (1846, PI. 21, fig. 6) they are not represented, 
but in figs. 123 and 135 of the present paper some of them are 
visible. These calicles, which are extremely small, are situated 
between two of the chief septa. Their centre is formed by the 
fusion of some of the smaller septa, a columella has not developed. 
In the regenerated specimens only these small calicles are to be 
found, except in one specimen in my material next to the line 
where the regeneration has begun. Here a number of somewhat 
larger calicles are present, but they are far more irregular than 
those of the specimen of fig. 135. 
The lower surface of the corallum has well-marked costae corre- 
sponding with the septa, and therefore radially arranged. The spines 
of the chief costae are large, blunt, often divided at the apex and 
always beset with thin ramifications. The chief costae alternate with 
some costae of less prominence. On the lower surface of the 
specimen of the U. S. National Museum a distinet scar of detach- 
ment is visible beneath the central calicle (fig. 134, cf. also 
Vaughan, 1905). 
This. species is at first sight easily to be distinguished from the 
other species of Halomitra. Its inconspieuous secondary calicles 
give a very even appearance to the upper surface of the corallum 
(figs. 123, 135). The costal spines show some resemblance with 
those in some specimens of H. robusta, but they are more separ- 
ately distributed (figs. 124, 125, 134) and on the whole the coral¬ 
lum is very light and fragile whilst the corallum in H. robusta is 
very heavy and solid. 
I have used in the above diagnosis the term „chief septa“ for 
the septa of the lower cycles and the marginal part of those of 
the higher cycles with which two septa of a next higher cycle have 
fused. In the specimen in which the central calicle is present the 
different cycles of septa can be easily distinguished for they are 
placed in more or less straight lines from the centre till the margin. 
Owing to the regeneration in the larger part of the specimens 
there are the usual abnormalities in the course of the septa: in 
some points the new formed septa have a direction at right angles 
to that of the septa in the original fragment. 
The generic distinetion of Zoopilus from Halomitra was already 
