PLATE XX. 



All the figures in this Plate illustrate the structure of Orbitoides. 



Fig. 



1 — Ideal representation of a half disk of 0. Fortisii, 

 laid open by vertical and horizontal sections, 

 showing the resemblance of its median cham- 

 bered plane to that of Cydoclypeus, and the in- 

 terposition of flattened charaberlets between the 

 lamellae enclosing them on either side: 20 diam. 



2. — Ideal representation of a small portion of a 

 similar disk, more highly magnified ; a, a, a, 

 interior of the chambers of the median plane ; 

 c, c, their passages of communication with the 

 chamberlets of the annuli internal and ex- 

 ternal to them ; d, d, oblique passages of 

 communication between the chamberlets of 

 the superficial layers ; e, e, non-tubular cones ; 

 h, h, canal system : 60 diam. 



3. — Section of the chambered structure of the 

 median plane, showing the septa to be com- 

 posed of two lamellae, with an interseptal 

 canal-system : 50 diam. 



4. — Section of the chambered structure of the 

 superficial plane, showing the irregular form 

 of the chamberlets, and the tubularity of the 

 horizontal partitions : 50 diam. 



5. — Section of the chambered structure of the 

 median plane of 0. Manlelli, showing the com- 

 munications of the chambers of successive 

 annuli by the oblique passages, a b, a b, and 

 showing also an appearance of communica- 

 tions between the adjacent chambers of the 

 same annuli at their inner margin : 60 diam. 



6. — Vertical section of 0. Mantelli, showing at a, a, 

 the chambered structure of the median plane ; 

 c, c, apertures of communication between its 

 chambers; d, d, oblique passages between the 

 chamberlets of the superficial layers; /, /, 

 interseptal canals : 60 diam. 



7. — Vertical section of a small specimen of 0. For- 

 tisii, showing a strong resemblance to a cor- 

 responding section of Nummulina : 20 diam. 



8. — Portion of section of O. MantelU, passing some- 

 what obliquely from the chambers of the 

 median plane, seen in the lower part of the 

 figure, to those of the superficial layer seen in 

 the upper part ; showing in the former the 

 lateral communications between the chambers : 

 50 diam. 



9.— A similar section from another part of the same 

 specimen, showing at its lower part a different 

 form of the chambers of the median plane 



Fig. 



10.— 



11.- 



11*. 



13.— 



li- 



15.— 



16.- 



and the interseptal canals between the two 

 lamellae of their septa, and showing at its 

 upper part the communications between the 

 chambers of the superficial layer : 50 diam. 



Vertical section of a large, thick specimen of 0. 

 Fortisii, showing the very small proportion 

 which its median chambered plane bears to 

 its superficial layers (that of the under side 

 having been partly broken a-^vay), and showing 

 the conical pillars of solid shell-substance, 

 traversed by canals : 10 diam. 



Portion of the same section more enlarged, show- 

 ing at e, e, one of the solid pillars, traversed 

 by brauches of the canal-system: 100 diam. 



—Vertical section of a very small specimen of 

 O. Mantelli, showing the very large size of its 

 primordial chamber, and the unusual height of 

 the charaberlets of its superficial layers: 20 diam. 



Section of the superficial layer of O. Fortisii, 

 taken parallel and near to the surface, show- 

 ing the verj' large size of the solid jiillars 

 e, e, e : 100 diam. 



Similar section, taken nearer the median plane 

 (being part of the section represented on a 

 smaller scale in fig. 14), showing the smaller 

 size of the pillars, and their penetration by 

 branches of the canal-system : 100 diam. 



Section of 0. Fortisii, taken in the same direc- 

 tion as the preceding, but much less enlarged, 

 and passing in part through the chambered 

 median plane, and in part through one of the 

 superficial layers ; in the former is seen the in- 

 completeness of several of the annuli : 20 diam. 



\ ertical section of a large, thick specimen of 

 O. Fortisii, showing the median chambered 

 plane divided transversely, the great thickness 

 of the layers by which it is invested, and the 

 prolongation of the superficial parts of these 

 so as to meet at the margin and thus com- 

 pletely to enclose the median portion : 16 diam. 



Portion of a similar section more enlarged, show- 

 ing the chambers a, a, a, of the median plane, 

 divided by double septa, between the lamellje 

 of which are seen indications of interseptal 

 canals ; above these are seen the piles of 

 flattened chamberlets of the superficial plane, 

 communicating by the oblique passages d, d, 

 and partly separated from each other by the 

 columns e, e, of solid shell-substance : 40 diam 



