GENUS ORBITOLITES: EXPLANATION OF PLATES. 235 



PLATE VII. 



Figs. 1 4. A series of specimens, drawn under the same magnifying power (35 

 diam.), illustrating the variations which present themselves in regard to 

 the Size of the nucleus. 

 The remaining figures represent various appearances exhibited by the Surfaces of 



Disks, simple and complex : all magnified 35 diam., except fig. 6, which is magnified 



70 diam. 



Figs. 5 7. Portions of surfaces of simple disks, in which the concentric circles an: 

 strongly marked, and the transverse divisions of the cells are compara- 

 tively obscure. 



Fig. 8. Simple disk, slightly concave, the marginal cells laid open by abrasion, the 

 surface marked rather by ' engine-turned' or excentric, than by concentric- 

 circles. 



Fig. 9. Portion of the surface of a complex disk, having small, round, and flattened 

 superficial cells. 



Fig. 10. Simple disk (from Australia), slightly concave, the marginal cells laid open 

 by abrasion, the surface irregularly thickened by calcareous deposits. 



Fig. 11. Flattened simple disk (from the Philippines), with abundant superficial depo- 

 sits, arranged with considerable regularity in a radiating direction. 



Fig. 12. Portion of the surface of a large complex disk, with very narrow oblong 

 superficial cells. 



Fig. 13. Portion of the surface of a complex disk, showing a marked difference in the 

 proportions of the straight-sided cells, even in adjacent zones. 



Fig. 14. Surface of simple disk (Cyclolmai of D'Orbigny), with an excentric nucleus, 

 surrounded by strongly-marked concentric circles. 



Fig. 15. Portion of the surface of a simple disk, showing unusually large and pro- 

 tuberant ovoidal cells. 



Fig. 16. Portion of the surface of a large fossil disk of complex type, showing varieties 

 in the size and form of the superficial cells. 



PLATE VIII. 



Fig. 1. Inner surface of a zone of a thick fossil disk, showing a tolerably regular 

 arrangement of the columnar cells, the segmental constrictions of these, 

 and the fissures leading into those of the next row : 35 diam. 



Fig. 2. Outer surface of a similar zone, showing the continuity of the cells of the 

 superficial layer with the vertical columns of the intermediate (the latter 

 not unfrequently dividing, however, at their extremities, so as to form two 

 cells at the end of each column), and the pores leading to the oblique 



