206 THE VOYAGE OF H.M.S. CHALLENGEE. 



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 laminae ; 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 rising 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 calicle, before reaching which they become lost (PI. XVI. fig. 6). They are exca- 

 vated below by a series of notches (PI. 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 laminae of the septa, by means of which 

 the septa are fused to the transverse trabecular connecting the costae already described 

 (PI. XVI. fig. 3). The free margins of the septa towards the margin of the disc are 

 slightly and irregularly 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 (PI. 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 

 figure. 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 (PI. 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 costae. The 

 bifurcations of the costae 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 costae commence growth at the centre of the disc 

 as six, and almost immediately become twelve by bifurcation. Two of the twelve costae 

 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 trabecule 



