150 THE VOYAGE OF H.M.S. CHALLENGER. 
marginal, but having become nearly obliterated by fusion with other tubercles successively 
formed outside them, and sometimes entirely lost in the resulting ridge. In one speci- 
men a second series of very delicate ridges is clearly marked radiating outwards for a 
short distance from the base of the central cone and corresponding with the tertiary and 
quaternary cost. The whole surface of the base of the corallum is covered with small 
rounded closely apposed granules. The large marginal tubercles of the base are from 4 
to 5 mm. long, and about 4 mm. broad at their origin from the base. Superiorly they 
are joined by slightly elevated rounded ridges, the continuations into them of the primary 
and secondary coste. The majority of the tubercles are tapering but some are obtuse. 
They terminate in three or four irregularly disposed spines. Two or even three marginal 
tubercles are sometimes fused together laterally into one mass. 
From the region of origin of the marginal tubercles the wall of the calicle slopes out- 
wards at an angle of about 60° with the plane of the base, its height above the plane 
being about 14 mm. The rounded ridges described as passing into the bases of the 
marginal tubercles reach upwards to the margin of the calicle. The external edges of the 
exsert primary and secondary septa are continued downwards along the middle of these 
costal ridges for about one-third the height of the wall of the calicle. The tertiary and 
quaternary coste are present as much smaller ridges, separated by fine vertical striz. 
The calicle is somewhat constricted at the region of attachment of the marginal spines, 
hence its lateral outline is not linear but curved slightly at a short distance from its 
inferior origin. The whole surface of the wall of the calicle is scattered over with small 
pointed granulations. 
The calicle, which is circular in outline, is shallow. The arrangement of the septa is 
irregular in the two smaller specimens, which are of the size of the one described by 
Pourtalés : there are six systems and four cycles. In the other three larger specimens, in 
several of the large interseptal spaces, included between the primary and secondary septa, 
two septa are developed in addition to the usual three. In the largest specimen these addi- 
tional septa are present in five of the twelve larger interseptal spaces, in another in six. In 
one intermediate in size between the two last mentioned, the primary and secondary septa 
combined are thirteen in number, and one of the larger interseptal spaces only has addi- 
tional septa developed in it. There is thus a tendency in this species to develop a fifth 
cycle of septa. A large number of the Trochocyathi armés of Milne-Edwards and Haime 
are provided with five cycles of septa. The septa are complete with the exception of 
those of the fourth and partial fifth cycle. The primary and secondary septa are very 
prominently exsert, projecting 4 mm. vertically above the margin of the wall of the calicle. 
Their edges are rounded, and they slope gradually downwards to the spot where the pali 
take origin. The tertiary and quaternary septa are also exsert, but in a much less degree. 
The tertiary septa occasionally coalesce with the primary or secondary. The primary, 
secondary, and tertiary septa are provided with pali which form three circlets. The pali 
