564 DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 



oblique stolon-processes, that pass off 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. III., 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., 6, 6) puts forth a set of stolon-processes from its entire periphery, each of 



Fier. 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 0. 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 



