EXPLANATION OF 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. 



Fig. 3. Part of the internal surface from another Operculina, 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 Operculina, passing through nearly the centre of the 

 spire ; showing at a, 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 with that of the preceding convolution near the margin of the 

 latter, the alar prolongations of the outer chambers being comparatively 

 small ; c, septum between contiguous chambers, traversed by the interseptal 

 system of canals; d, d, marginal cord divided transversely (see figs. 15, 16); 

 e, e, fissure of communication between contiguous chambers ; f,f, spiral canals 

 divided 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 ; b, 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, a, marginal cord traversed by canal-system ; b, 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; d, d, 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- 



