206 THE VOYAGE OF H.M.S. CHALLENGER. 
here and there in patches. The septa are in six systems and four cycles, with part of a 
fifth cycle. The primaries are free from the periphery to the columella, and straight, and 
consist of very thin laminz ; the secondaries are free to near the columella, where they 
are covered over by the fused tertiaries, which in joining form a thin laminar expansion 
over them. The tertiaries bifurcate at a short distance from their junctions, which is 
equal in all the systems, and the two septa thus derived from each of them themselves 
bifurcate at a further distance again from the columella, which distance also corresponds 
symmetrically in all the systems. At each of the bifurcations there is a certain small 
amount of laminar hard tissue developed covering over the interval in the fork. In 
each system one of the four septa derived from this last-mentioned bifurcation, namely, 
that which lies second away from each primary septum, has developed on its side next 
the secondary septum a small and short additional septum (PI. XVI. figs. 3, 4). The 
septa are extremely low, their lamella not risimg more than 1 mm. above the level of the 
base of the corallum near its centre, and gradually becoming lower towards the margin 
of the ealicle, before reaching which they become lost (Pl. XVI. fig. 6). They are exca- 
vated below by a series of notches (Pl. XVI. fig. 7), which correspond to the perforations 
in the wall of the base, and are seen from beneath in Plate XVI. fig. 5. Between these 
notches are a series of short processes of the laminee of the septa, by means of which 
the septa are fused to the transverse trabeculee connecting the coste already described 
(Pl. XVI. fig. 3). The free margins of the septa towards the margin of the disc are 
slightly and irreeularly dentate. Towards the columella they bear a series of long but 
very delicate spines, which are all directed outwards at an angle, and are arranged on 
the septa at definite intervals with great regularity (Pl. XVI. figs. 2 and 7), increasing in 
size towards the margin of the disc. Each of the primaries bears six of these spines, 
each of the secondaries five, three of which project through the laminar expansion found 
at the junction of the tertiaries over the secondaries. There is a spine at the point of 
bifurcation of the tertiaries, and other spines disposed close to these as shown in the 
fieure. A few very small and short similar spines compose the columella. The whole 
group of spines is confined to the central region of the disc, and is seen to rise from it 
like a cluster of small prickles when the corallum is viewed edge-wise (Pl. XIV. fig. 3). 
The septa correspond in position with the intervals between the long spikes into 
which the margin of the disc is prolonged, and to the intervals between the coste. The 
bifurcations of the coste and those of the septa above them do not correspond in position 
or arrangement, as will be seen from Plate XVI. fig. 1. The primary and secondary 
septa do not branch at all, whereas the costze commence growth at the centre of the disc 
as six, and almost immediately become twelve by bifurcation. Two of the twelve coste 
thus derived in each system, those next the primaries, soon branch again, whilst the pair 
accompanying the secondary septa do not branch for a considerable distance further away 
from the centre of the disc. By branchings at successive intervals four costal trabeculze 
