PLATE VI. 



Structure of the Calcareous Disk of Orhitolitcs complanata, as seen by retlected light. 



Figs. 1-3. — Three young typical specimens, showing the large size of the " nucleus " (originating 

 in the primordial chamber a), and the completeness of even the very earliest annuli of chamberlets. 

 Magnified 35 diameters. 



Fig. 4. — Ideal representation of a typical disk, laid open in various modes to show its interior 

 structure : — «, primordial chamber ; h, circumambient chamber ; c, c, concentric annuli of oblong 

 superBcial chamberlets ; d, marginal pores of peripheral annulus ; (/', d", corresponding pores of inner 

 annuli, once marginal, but now connecting the interior with exterior annuli; c', c', vertical section in 

 radial direction, showing intermediate stratum distinct from superficial layers; /,/, floors of superficial 

 chamberlets, with an aperture at either end of each ; r/, g, annular canals running beneath tliese floors, 

 with large apertures leading to the columnar chamberlets of the intermediate stratum ; ;/, similar 

 canals near the other surface of the disk ; ;^", similar canals laid open through the plane on which 

 they give off the two passages to cash superficial chamberlet; g'", annular canals cut through in 

 vertical section; A, passage of horizontal section through summit of intermediate stratum, sliowing the 

 tops of the columnar chamberlets ; i, i, i, and k; l\ /.-, passage of horizontal section through two 

 different planes of intermediate stratum, showing connection between columnar cliamberlets of succes- 

 sive zones, by oblique passages running in opposite directions. 



Fig. 5. — Vertical section, taken in the radial direction, of a " sub-typical" example, wliose earlier 

 development has taken place on the " simple" plan : — «, cavity of "nucleus" ; from aa to hh the annular 

 canal single in each zone ; from Ih to cc the annular canals double in each zone, and separated from 

 each otlier by an interposed stratum, the cliamberlets of which are generally continuous with those of 

 the superficial planes, much irregularity in this respect showing itself between cc and dd. Magnified 

 50 diameters. 



Fig. 6. — Central portion of a disk, the first three zones of which, between aa and hh, are formed 

 upon the "simple" type, each having but a single annular canal and one row of septal passages; at 

 hh the " complex " type is assumed, the annular canals being doubled, and separated liy an interposed 

 stratum ; but the columnar chamberlets of this stratum that lie between the annular canals are 

 continuous from hh to cc with the chamberlets of the superficial layers, this continuity giving place 

 between cc and dd to the alternation in their positions characteristic of the most typical Urbitoliuo 

 disks. Magnified 50 diameters. 



Fig. 7. — Tangential vertical section of a typical disk, taken near the thick margin ; showing the 

 two layers of superficial chamberlets, the double series of annular canals, and the interposed stratum 

 traversed by the radial passages that open on the periphery as marginal pores. Magnified 50 

 diameters. 



Fig. 8. — Vertical section of a disk, of which the first five zones, aa to hh, are formed on the 

 "duplex" type, each having but a single annular canal, but a double series of radial passages; 

 between hh and cc the annular canals are double, and the chamberlets of the interposed stratum are 

 continuous with the superficial chamberlets ; while from cc to dd the superficial chamberlets alternate 

 in position with those of the interposed stratum. Magnified 48 diameters. 



Fig. 9. — Vertical section of inner part of typical disk, showing the circumambient chamber, aa, 

 immediately surrounded by annuli of the complex type, having the two superficial layers of 

 chamberlets, /', h, completely dissociated from the columnar chamberlets, c, c, of the interposed stratum. 

 Magnified 48 diameters. 



Fig. 10. — Vertical section of inner part of typical disk, showing the circumambient cliamber, a, 

 communicating by only a single passage with the " simple " chamberlets of the first annulus h ; but 

 this at once passing at c, by the doubling of the annular canal, into the "complex," which is thence- 

 forth maintained, c-d, with a progressive increase in the thickness of the disk. ]\Iagnified 48 

 diameters. 



