EXPLANATION OP THE PLATES. 
39 
the neighbourhood of these, and the diminution in the size of the tubuli as 
they pass towards the outer surface : — 120 diam. 
Fio-. 3. Part of the internal surface from another Ojperculina, presenting smaller and 
more numerous tubercles on its exterior: — 120 diam. 
Fig. 4. Section through the wall of a contiguous chamber of the same : — 120 diam. 
Fig. 5. Section through the median plane of the earlier convolutions of an Operculina, 
showing the spheroidal form of the primordial chamber, the irregular forms 
of the chambers that immediately succeed it, and one of the spiral canals, 
which may be traced in the marginal cord almost to the centre of the spire; — 
75 diam. 
Fig. 6. Portion of the external surface of an Operculina, of which the tubercles were 
particularly prominent: — 75 diam. 
Fig. 7. Vertical section of the inner convolutions of an Operculina, which has happened 
to pass close to the primordial chamber and the one that springs from it, 
and which shows at a the two spiral canals running along their exterior : — 
100 diam. 
Fig. 8. Vertical section of a typical Operciilma, passing through nearly the centre of the 
spire ; shonung at «, a the spiral lamina of the second whorl, separated from 
that of the whorl which it invests by the alar prolongations of the chambers 
of the latter, which extend nearly to the centre of the spire ; b, the spiral 
lamina of the last convolution, thinned away in proportion to its extension, 
and coalescing ndth that of the preceding convolution near the margin of the 
latter, the alai’ prolongations of the outer chambers being comparatively 
small ; c, septum between contiguous chambers, traversed by the interseptal 
system of canals; fZ, fZ, marginal cord divided transversely (see figs. 15, 16); 
e, e, fissure of communication between contiguous chambers ; /,/, spiral canals 
dirided transversely : — 40 diam. 
Fig. 9. Very thin section of a lamina of the tubular shell-substance of Operculina, 
magnified 120 diameters, cutting the tubuli transversely, and showing a 
delicate areolation between them ; h, prismatic appearance of a portion of the 
same, magnified 250 diameters ; a, another portion, magnified 250 diameters, 
showing the comparatively large size of its tubuli close to the internal surface. 
Fig. 10. Portion of a section of a typical Operculina (see Plate VI. fig. 3), taken through 
the median plane : — a, «, marginal cord traversed by canal-system ; 5, b, walls 
of the chambers, composed of minutely-tubular substance; c, septal bands 
with their lateral branches, composed of transparent non-tubular shell-sub- 
stance ; fZ, fZ, one of the spiral canals : — 50 diam. 
Fig. 11. Portion of a similar section of a tuberculated specimen of Operculina, showing 
the clear spaces left by the absence of tubuli in the spots beneath the tuber- 
cles, and the semi-opacity produced by the crowding together of the tubuli 
elsewhere; — 50 diam. 
Fig. 12. Portion of a vertical section passing tangentially along the exterior of the inner- 
