32 THE VOYAGE OF H.M.S. CHALLENGER. 



or even less, to nearly 1 inch, and a thickness of from 0'012 inch to O'lOO inch. This range 

 is less, however, in the disks brought up in the 18 fathoms' dredging, than in those 

 collected nearer the surface, the average diameter of what seem to be the adult forms in 

 the former not exceeding 0'7 inch ; and there is an almost entire absence in them of 

 those irregular outgrowths which are frequent in the large disks found on the summit of 

 the reef. Tlie disks are sometimes almost plane, with a slight central depression ; but 

 are more commonly decidedly biconcave. The central portion, consisting of the "nucleus" 

 and the annuli that immediately surround it, is almost invariably the thinnest, and 

 round this there is usually a progressive increase in the thickness of the next succeeding 

 annuli. If this increase continues, the disk of course becomes thickest at the margin ; 

 but it not unfrequently ceases, so that the rest of the disk is plane ; and sometimes, at 

 about half the distance between the centre and the circumference, the thickness of each 

 succeeding annulus diminishes, so that the marginal portion of the disk is no thicker than 

 the central. 



The concentric bands into which each surface of the disk (PI. VI. fig 4) is marked 

 out, are complete in the typical forms of this species, to the very margin of the nucleus ; 

 not the least vestige being here seen of any "orbiculine" spiral, l)ut the cyclical plan of 

 growth characteristic of the Orbitoline t}^e being exhibited from the very commence- 

 ment. The breadth of each zone averages about 0"003 inch, and the number of zones 

 bears a pretty uniform relation to the diameter of the disk. In one of the largest disks 

 that I have examined there are 166 zones, while the smallest has only three. Each zone 

 is crossed by radial lines, which mark out areolae that are usually somewhat rectangular 

 in shape and sometimes approach a square, but are more commonly at least twice as 

 long (in the radial direction) as they are broad, their long sides being nearly parallel to 

 each other. The margin does not usually show any such convexities as are formed in 

 Orhitolites marginalis by the projection of the columnar chamberlets ; but the marginal 

 pores are usually arranged more or less regularly in vertical rows, which are, however, 

 often incomplete, — the two adjacent rows, in such cases, usually inclining towards 

 each other. There is no constancy in the number of pores in the different vertical 

 rows of even the same annulus; and there is no such regularity in their disposition as would 

 mark out a horizontal stratification. 



The "nucleus" is much larger in the typical forms of this species than in either of the 

 preceding ; and though it exhibits a considerable range of dimension, as shown in PI. VI. 

 figs. 1, 2, 3, yet even the smallest nuclei of those disks whose innermost annuli are 

 formed on the " complex " plan are many times larger than those of Orhitolites duplex. 

 Its two surfaces are generally flat, or nearly so, but are sometimes slightly convex. 

 The pyriform primordial chamber a, as in Orhitolites marginalis and Orhitolites du2)lex, is 

 surrounded by a large "circumambient" chamber ; and this usually shows a partial division 

 by an incomplete partition. 



