PLATE IV. 



Structure of Calcareous Disks of Orhitolites marginalis (figs 1-5), and Orbitolites duplex 



(iigs. 6-10), as seen by transmitted liglit.^ 



Orbitolites marginalis. 



Fig. 1. — Tliin disk, mounted in Canada balsam, showing the general arrangement of its concentric 

 annuli aronnd an excentric " nucleus " and orbiculine interior. Magnified .30 diameters. 



Fig. 2. — Interior portion of the same. Magnified 64 diameters. 



Fig. 3. — Peripheral portion of the same. Magnified 64 diameters. 



Fig. 4. — Thin section of inner portion of a disk, showing the communications between its 

 chamberlets : — a, primordial chamber; i, circumambient chamber, leading by a single passage into 

 next chamber, c, which opens by three radial passages into as many chamberlets forming the first 

 zone d, and these into the chamberlets of the next zone c, the chamberlets of each zone communi- 

 cating with each other laterally. The radial passages open at the outer side of each zone, as marginal 

 pores,/,/. Magnified 90 diameters. 



Fig. 5. — Sarcodic body occupying inner portion of disk ; — a, primordial segment giving off h, 

 circumambient segment, and this giving ofif the single segment c, from which proceed the stolon- 

 processes that form the first imperfect zone of sub-segments d ; from this, again, are given off the 

 stolon-processes that form the more complete zone c ; and each zone increases in length, until the 

 ninth and tenth completely enclose the circumambient segment. At/,/ are seen the sarcodic annuli 

 which connect together the chamberlets of each zone, and the radial stolon-processes that issue from 

 this to form the next annulus. Magnified 90 diameters. 



Orhitolites duplex. 



Fig. 6. — Section of disk through superficial plane, showing its " engine-turned " aspect. Magni- 

 fied 16 diameters. 



Fig. 7. — Section through median plane, showing annular canals laid open, and the large passages 

 for the pedicles of the lower series of columnar sub-segments. Magnified 50 diameters. 



Fig. 8. — Section through a somewhat deeper stratum, showing the successive septa, s^-s^, that 

 divide the annular canals, with the columnar chamberlets r, c, into which they open beneath, and 

 the oblique stolon-passages st, st, which pass to the chamberlets of each zone from the annular canal of 

 the preceding zone. Magnified .50 diameters. 



Fig. 9. — Surface-aspect of central portion of disk, showing at a the primordial chamber, and at h 

 the circumambient chamber, the first-formed zones around which are irregular and incomplete. Magni- 

 fied 04 diameters. 



Fig. 10. — Section of central portion of disk through median plane, showing at a the primordial 

 chamber, and at h, h the circumambient chambers, from one side only of which last are given off the 

 stolon-passages that dilate into the first imperfect zone of chamberlets. The successive zones, 

 however, complete themselves so as to enclose the " nucleus " much more speedily than in Orhitolites 

 niarghudis ; so that the " nucleus " has but a slight excentricity. Magnified 90 diameters. 



