564 
DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
oblique stolon-processes, that pass oft alternately at regular intervals from the two 
sides of each column, traversing the annular septa ; and the orifices of the passages 
in the last-formed septum, through which these stolon-processes extend themselves 
outwards, are seen as multiple series of pores on the margin of the disk (fig. Ill,, 1). 
The vertical section of the calcareous disk given in fig. III., 2, shows the separation 
of the two superficial planes of chamberlets by the interposition of the shelly fabric 
that gives lodgment to the intermediate sarcodic columns; while at 3 is shown 
diagrammatically, on a larger scale, the cavitary system of the disk, with the commu¬ 
nication between its several parts. At a are seen the chamberlets of the superficial 
planes, which are completely closed in when not abraded; and these are shown in 
vertical section, above and below, at b, while at c are seen their floors, each having a 
pore at either end, which communicates with the annular canal beneath. The annular 
canals are seen at d in vertical section, and at d' and d" as laid open in horizontal 
section; the former showing how they cross the tops of the cylindrical chamberlets of 
the intermediate stratum, and the latter (taken a little nearer the surface) showing 
the manner in which they open into the pores leading to the superficial chamberlets. 
In the lower part of the figure, the intermediate stratum is traversed by two horizontal 
sections in slightly different planes, cutting across the cylindrical chamberlets, and 
showing the two series of oblique stolon-passages by which the chamberlets of 
successive annuli communicate with each other. 
The nuclear mass which occupies the centre of the disk consists, as in 0. duplex, 
of a “ primordial segment,” surrounded by a “ circumambient segment,” and this last 
(fig. V., b, b) puts forth a set of stolon-processes from its entire periphery, each of 
Fig. V. 
which gives origin to a columnar sub-segment; so that a complete annulus is at once 
constituted, thus establishing the cyclical plan of growth from the very first. 
The collection of specimens of O. complanata made on the Fiji reef contains disks 
of all sizes ranging from 0 - 04 inch to nearly 1 inch; and even in the smallest of 
them, whose nucleus is surrounded by only two or three annuli, the immediate 
assumption of the completed plan is marked by the multiplicity of the series of 
marginal pores. But while this may, I think, be unquestionably regarded as the 
typical condition of the species, the collection also includes an abundance of disks 
whose peripheral portion is characteristically “ complex,” whilst their central portion 
is no less characteristically “simple ; ” the passage from the one plan of growth to the 
