GENUS ORBITOLITES: — EXPLANATION OF PliATES. 
233 
type ; from i to c the annular canal double, and an irregular intermediate 
stratum interposed, from which, however, the superficial cells are not com- 
pletely differentiated ; and from ciod the imperfect separation of the super- 
ficial cells from the intermediate layer, and the extreme irregularity of the 
latter: — 50 diam. 
Fig. 8. Tangential section, near the margin of the same disk, showing the same 
peculiarities : — 50 diam. 
Fig. 9. Central portion of a disk, showing the nuclear cavity divided into two (the 
section having traversed the central cell and one side of the circumambient) ; 
the formation of the first three zones {a — h) on the simple type ; the im- 
perfect separation of the superficial cells from the intermediate stratum in 
the next two zones {h — c) ; and the regular development of the remainder 
(c — d) on the complex type: — 50 diam. 
Fig. 10. Vertical section of the central portion of a disk of complex type; showing 
four cavities in the nucleus ; the first five zones {a — h) constructed upon 
the simple type, with a single annular canal, but an incipient separation 
manifesting itself between the superficial cells and the intermediate stratum ; 
in the next three zones {h — c) the annular canal double, but the superficial 
cells still partly continuous with the columnar; the outer part (c — d) 
framed according to the regular complex type: — 48 diam. 
Fig. 1 1. Vertical section of a disk of complex type, showing an unusual development 
of vertically superposed cells immediately around, and even partly cover- 
ing, the nucleus, from which they arise by four layers of passages ; a pro- 
gressive diminution in the thickness of the disk, as far as the fifth zone {h), 
with a gradual approximation towards the regular type, on which the 
remainder {h — c) is developed : — 48 diam. 
Fig. 12. Vertical section of a disk of complex type, showing commencement of its first 
zone by a single layer of passages {a) from the nucleus ; the presence of 
two annular canals, with the absence of separation of the superficial cells, 
in the first zone {h ) ; the incomplete separation of the superficial cells from 
the intermediate columnar portion in the second zone (c) ; and the normal 
conformation of the remainder (c—d) : — 48 diam. 
PLATE VI. 
Fig. 1. Portion of a disk of simple type, from the immediate neighbourhood of the 
nucleus, showing the beak-like projections at the sides of the passages very 
remarkably developed : — 100 diam. 
Fig. 2. Horizontal section of the intermediate layer, passing (in consequence of the 
flexure of the disk) through two different planes, and thus showing the 
two different directions of the oblique passages which connect the colum- 
