EXPLANATION OF PLATES. 
567 
Fig. 20. Inner surface of one of the annuli of a similar fossil disk, showing the differen- 
tiation of the superficial from the intermediate layers of cells : — 50 diam. 
Figs. 21, 22. Portions of the superficies from similar fossil disks, showing varieties in 
the surface-markings : — 50 diam. 
Fig. 23. External aspect of Alveolina Boscii (recent); a, a, growing margin, showing 
multiple apertures resembling those at the margins of Orhitolites and Orhi- 
cuUna: — 40 diam. 
Fig. 24. Longitudinal section of Alveolina Boscii, showing its internal structure and 
the successive stages of its growth : — 40 diam. 
PLATE XXIX. 
Fig. 1. Section of disk of Orhiculina adunca parallel to the surface, showing the cells 
and their communications : — 100 diam. 
Fig. 2. Surface-layer of disk of Orhiculina adunca, showing its punctuated appear- 
ance : — 100 diam. 
Fig. 3. Vertical section of disk of Orhiculina adunca, passing through its central 
nucleus, and showing columnar arrangement of its cells, and the manner in 
which the earlier whorls of the spire are invested by the later : — 100 diam. 
Figs. 4-7. Transv^erse sections of Alveolina Boscii, showing the increase in the num- 
ber of turns of the spire from its terminal to its central portion : — 40 diam. 
Fig. 8. Portion of a similar transverse section enlarged; showing — a, a, a, internal 
prolongations of the surface-layer ; h, h, h, outer longitudinal canals ; 
c, c, c, inner longitudinal canals : — 80 diam. 
Fig. 9. Portion of a longitudinal section (Plate XXVIII. fig. 24) similarly enlarged ; 
showing 1, 2, 3, 4 successive layers formed by the involution of the spire; 
and in each the passages a a, h h, and c c, between one band and the 
next. (N.B. The variation in the appearances presented by the other 
layers, depends upon the difference of relative direction in which the sec- 
tion traverses each of them respectively) : — 80 diam. 
Fig. 10. Thin section of Cycloclypeus, taken parallel to the surface, and close to the 
covering of the chambers ; showing part of the system of interseptal canals : 
— 50 diam. 
Fig. 11. Diagram of a single chamber of Cycloclypeus, showing its relations to other 
chambers, and to the interseptal system of canals: a, cavity of chamber; 
h, h', adjacent chambers of the same annulus, each separated from a by a 
double septum; cdanddd', chambers of internal and external annuli, 
separated from a by the annular partitions ee, e' e', but communicating with 
it by the passages /, f, f, /*; in the septa between a and h, V are seen the 
interseptal canals, each of which sends two oblique branches across the 
annular septa, to communicate with corresponding canals in the septa 
