152 



THE VOYAGE OF H.M.S. CHALLENGER. 



Stephanotrockus diadema, Moseley (PL III. fig. 1, a-c). 



Geratutrochus diadema, Moseley, Proc. Roy. Soc, 1876, p. 553. 



The coralluni is white and saucer-shaped. The central region of the base is flattened 

 and nearly horizontal, the lateral portion of the wall rises with some abruptness from this 

 horizontal region at an angle of about 55° with the vertical. A very short rudimentary 

 pedicle is seen in the centre of the base and terminates in a small scar of adberence. 



Stephanotrockus diadema. (Once and a half the natural size.) 



From the base of the pedicle radiate out well-marked costal ridges corresponding to the 

 primary and secondary septa. These ridges are sharp and serrate, the dentations being 

 inclined towards the margin of the calix. The primary costa are more prominent than 

 the secondary, and take origin nearer the peduncle. Tertiary costse are present as only 

 slightly elevated narrow untoothed ridges, most marked near the margin of the calyx ; the 

 quaternary costa are very faintly marked. There are six systems of septa and five cycles. 

 The whole of the septa are exsert, the primary and secondary extremely so, projecting 

 nearly a centimeter above the margin of the calicle ; the quinary septa next to the primary 

 and secondary are higher than the quaternary and equal in height to the tertiary, in some 

 instances even higher than these latter ; they are joined for nearly their entire height to 

 the primary and secondary septa by prolongations of the wall. The quinary septa next 

 the tertiaries are lower than the quaternaries. The primary, secondary, and tertiary 

 septa are remarkably stout and straight, and rise above the level of the remaining septa 

 within the calicle. The septa are irregularly denticulate on their edges, and covered with 

 granules arranged in curved lines of growth ; the upper terminations of the exsert septa 

 are rounded. The free borders of the primary, secondary, and tertiary septa in their curved 

 sweep towards the columella each present a shallow notch, corresponding in position to the 

 region where the somewhat abrupt change from horizontal to inclined contour in the base 

 of the corallum occurs. Beyond this notch the border of each septum rises again slightly, 

 an indistinct indication of a paliform lobe being thus formed ; the primary septa are 

 entirely free from adhesion with others, and are thereby conspicuous ; the tertiary septa 

 are fused to the secondary close to the columella ; the quaternary unite with the tertiary 



