PLATE III. 



Structure of Calcareous Disks of Orhitolites marginalis (figs. 1-7), and Orhitolites duplex 



(figs. 8-14), as seen by reflected light. 



Orhitolites marginalis. 



Fig. 1. Surface of disk. Magnified 16 diameters. 



Fig. 2. Inner portion of the same, showing its excentric "nucleus " and the orbiculine arrangement 

 of its earlier zones of chamberlets. Magnified 64 diameters. 



Fig. 3. Peripheral portion of the same, viewed somewhat obliquely, so as to show at a the 

 columnar arrangement of the margin. Magnified 64 diameters. 



Fig. 4. Marginal view of two disks, a thin, h thick ; showing the marginal pores elongated 

 vertically, some of those in h being traversed by shelly bridges, which do not, however, completely 

 divide them. Magnified 64 diameters. 



Fig. 5. Interior view of a portion of an annulus separated by fracture from that which it 

 enclosed ; showing the vertically-elongated radial passages opening into its chamberlets. Magnified 64 

 diameters. 



Fig. 6. Vertical section, taken in radial direction, of peripheral portion of disk, showing the 

 single annular canal of each annulus, and the arcuate direction of the chamberlets. Magnified 64 

 diameters. 



Fig. 7. Peripheral portion of disk, of which tlie upper surface has been ground away, so as to lay 

 open the columnar chamberlets, the walls of whose last annulus form the fluted margin a. Magnified 

 64 diameters. 



Orhitolites duplex. 



Fig. 8. Surface of disk. Magnified 16 diameters. 



Fig. 9 Horizontal section of a disk, taken beneath the plane of the annular canals, showing at a 

 the openings into the lower series of columnar chamberlets, crossed by the annular septa ; and at h 

 the deeper plane from which the median stratum has been entirely removed. Magnified 50 diameters. 



Fig. 10. Peripheral portion of the surface. Magnified 50 diameters. 



Fig. 11 Horizontal section through the median stratum traversed by the annular canals, which 

 are separated by the successive septa s^-s^ ; on the convex sides of these septa are seen the large 

 passages leading obliquely downwards into the columnar chamberlets {see fig. 9) of their own lower 

 series, while on the concave sides of the septa are seen the small radial passages that pass from each 

 annular canal into the columnar chamberlets of the next annulus. Magnified 50 diameters. 



Fig. 12. Portion of disk laid open by removal of its upper surface, showing upper series of 

 columnar chamberlets, with the pore in every one, which is the opening of the oblique radial passage 

 from the annular canal of the preceding annulus. Magnified 50 diameters. 



Fig. 13. Margin of dislv, showing the marginal pores arranged in two series, generally alternat- 

 ing in position, and separated by elevated ridges. Magnified 64 diameters. 



Fig. 14. Central portion of disk, showing small " nucleus " and imperfect annulation of first-formed 

 zones. Magnified 64 diameters. 



