PLATE IX. 



All the figures in this Plate illustrate the structure of the calcareous disks of Orbitolites ; and the same 

 letters are used to mark corresponding parts throughout. 



a. Primordial chamber. 

 b b V . Circumambient chamber. 

 c, c. Chamberlets of the concentric zones ; c , c , c' , c' , the same, as seen in vertical section. 

 d. Passage from the primordial to the circumambient chamber. 

 e, e, e. Radiating passages, connecting the chamberlets of successive zones. 



f,f,f. Marginal pores on the external surface of the last annulus, in which the radiating passages terminate. 

 (/, g, (/. Inner surface of annulus separated by fracture from the outer surface of that to which it was applied, 

 showing the incompleteness on their inner side of the proper walls of the chamberlets, into each 

 of which a wide fissure is seen to open. 

 /(, /*, h. Annular galleries, connecting the chamberlets of their respective zones ; where the galleries are double, 

 as in figs. 7, 8, 9, A', /(', /(', point to the second series, and the space between the two constitutes 

 the intermediate stratum. 

 ■ i, i, i. Superficial chamberlets, with the two passages at their extremities ; i', i', i', the same as seen in vertical 

 section. 



Fig. 

 1.- 



Ideal representation of a disk of the simple type, 

 the details of the different parts made up from 

 actual specimens ; showing the natural sur- 

 face, with the external indications of the con- 

 centric zones of transversely oval chamberlets ; 

 the natural margin, with the single row of 

 pores /, /, between the protuberances of the 

 chamberlets ; a portion of the interior c, c, 

 e, e, as displayed by a (horizontal) section 

 parallel to one of the surfaces ; another por- 

 tion c', c' , as displayed by a (vertical) section 

 perpendicular to this, passing in a radial 

 direction ; and another portion g, g, as dis- 

 played by a fracture following the circular 

 course of one of the zones. 

 2. — Vertical section of a disk of simple type : 

 30 diam. 



section of a disk of simple type, 

 the excentricity of its early growth : 



section of a disk of simple type, 

 a regularly concentric growth from 



of simple type, 

 commencement : 



3. — Horizontal 

 showing 

 35 diam, 



i. — Horizontal 

 showing 



very large central and circumambient cham- 

 bers : 35 diam. 



5, — Horizontal section of a disk 

 showing a regularly spiral 

 90 diam. 



6. — Portion of a horizontal section of a disk of 

 simple type, enlarged to show the mode of 

 connection of the chamberlets c, c, c, by 

 annular galleries h, h, h, from which come off 

 the radiating passages e, e, e, that lead to the 

 succeeding zones ; at c", c" are seen two in- 

 terpolated chamberlets, whose radiating pas- 

 sages come off from chamberlets of the zone 

 within : 120 diam. 



7. — Ideal representation of a disk of complex type 

 (a portion of which is shown on a larger scale 

 in fig. 9), the details of the different parts 



Fig. 



made up from actual specimens ; showing the 

 natural surface, with the external indications 

 of concentric zones of narrow, rectilineal 

 chamberlets ; the natural margin, with its 

 multiple rows of pores lying in furrows /, /, 

 between the protuberances of the chamberlets ; 

 horizontal sections of the interior through 

 different planes, so as to lay open, at i, i, 

 the superficial chamberlets, at h, h, h, the 

 annular galleries into which these open be- 

 neath, at e, e, e, the cylindrical chamberlets of 

 the intermediate stratum, connected by the 

 radiating passages, and at h' h' , the lower set 

 of annular galleries ; at c , c', c , a vertical 

 section in the radial direction ; and at g, g, g, 

 the inner surface of an annulus, separated by 

 concentric fracture from that to which it was 

 applied. 



8. — Vertical section of a disk of complex type : 

 k, first diflerentiation of superficial chamber- 

 lets from intermediate stratum ; /, summits of 

 interzonal partitions, forming the floors of the 

 superficial chamberlets ; m, m, passages of 

 communication between the superficial cham- 

 berlets and the annular galleries ; n, first dupli- 

 cation of the annular galleries, and differen- 

 tiation of the intermediate stratum : 30 diara. 



9. — Portion of fig. 7, more enlarged. 



10. — Marginal surface of a thick fossil disk, showing 

 the rows of pores /, /, lying in the furrows 

 between the protuberances of the cylindrical 

 chamberlets, the irregular junctions and subdi- 

 visions of these, and the incomplete diffe- 

 rentiation of the superficial chamberlets from 

 the intermediate stratum : 35 diam. 



11. — Inner surface of a zone from the same disk, 

 separated by concentric fracture from that 

 which it enclosed : 35 diam. 



