REPORT ON THE GENUS ORBITOLITES. 23 



and every new annulus that is afterwards formed, exhibits exactly the same arrangement. 

 As a general rule, the chamberlets in each annulus alternate in position with those of the 

 annuli internal and external to them ; the radial passages which lead to them from the 

 preceding annulus having their origin, not in its chamberlets, but in the annular passao-e 

 that connects them. And it is only when an additional chamberlet is interpolated, in 

 accordance with the increased diameter of the added ring, that the passage leadino- to it 

 comes off directly from a chamberlet of the previous one. And thus it comes to pass 

 that the pores seen along the margin of the disk (PI. III. fig. 4) open hetiveen the 

 columnar chamberlets, each of them communicating with the chamberlet on either side 

 of it, as shown at/,/, PI. IV. fig. 4. 



The meaning of these arrangements is made clear by reference to PI. IV. fig. 5, which 

 shows the sarcodic body of Orhitolites marginalis, obtained by the solution of its calcareous 

 shell by dilute acid. The primordial segment a communicates by a narrow pedicle or 

 stolon-process with the circumambient segment h, and this, again, by a similar pedicle 

 with the segment c, which answers to the segment a, PI. II. fig. 1. From this are given 

 off the two radial pedicles that enlarge into the two sub-segments d; and these are united 

 laterally by a pedicle, that gives ofi" the radial extension which enlarges into the sub- 

 segment e of the next band. The same plan is maintained through each successive 

 addition, the sub-segments of each row showing themselves as enlargements of a 

 continuous cord of sarcode, on which they are threaded, as it were, like beads upon a 

 string. Each row of sub-segments represents the entii-e segment which occupies the 

 undivided chamber of a Peneroplis ; and so, when the first annulus is completed by the 

 meeting of the two extremities of that cord, it has still the same equivalent, which is, of 

 course, equally to be recognised in aU subsequent annuli. In the outer portions of the 

 disk of Orhitolites marginalis, the sub-segments acquire a columnar form by vertical 

 growth, which is in striking contrast with their extreme flattening in Orhitolites 

 tenuissimus. 



Thus, not only in the "orbiculine" stage, but throughout the whole later growth of 

 the disk in this type, we recognise the same essential features as in the preceding ; — the 

 subdivision of the " peneropline " chambers into chamberlets, and of the segments of the 

 body into sub-segments, taking place on precisely the same plan in both, and exactly the 

 same system of communications being maintained between the subdivisions ; — the only 

 difierence being in the form of these subdivisions, which is obviously a character of 

 comparatively trivial import. The question now suggests itself, what is the relation 

 between the "spiroloculiue" shell of Orhitolites tenuissima, and the "nucleus," consisting 

 of "primordial chamber" and "circumambient chamber," of Orhitolites marginalis. 

 This will be best answered by comparing the sarcodic bodies of the two types; for whilst 

 the small primordial segment of the one gives off a long, slender, slightly interrupted cord, 

 which coils round it several times before it begins to expand (PI. II. fig. 1), the large prim or- 



