PLATE XVII. 



All the figures in this Plate illustrate the structure of Operculina complanata. 



1. Ideal figure of Operculina, laid open to show its 



structure : a, a, a, marginal cord, divided 

 transversely at a', so as to show the orifices 

 of its canals, the distribution of which is seen 

 at a" a" in tangential section, and at a" a" 

 in section through the median plane ; b, b, b, ex- 

 ternal surface of the chambers, marked out by 

 the septal bands ; c, c, interior of the chambers 

 of the outer whorl, the alar prolongations of 

 which extend, as shown at c', c', over the sur- 

 face of the inner whorl, towards the centre of 

 the spire ; d, d, septa dividing the chambers, 

 formed by two lamellae, between which lie the 

 interseptal canals, whose smaller branches are 

 seen irregularly divided in the septa d' , d', while 

 in the septum d", d,'' one of the principal 

 trunks is laid open through its whole length ; 

 at the approach of each septum to the marginal 

 whorl is seen the fissure, e, e, e, e', which 

 forms the principal communication between the 

 chambers ; /, /, the secondary orifices ; g, g, 

 the distribution of the interseptal system of 

 canals, which branch from the two spiral 

 canals h, h ; and. at i, i are seen the conoidal 

 columns of nou-tubnlar shell-substance that 

 overlie the septa, forming tubercular projec- 

 tions on the surface, and frequently penetrated 

 by branches of the interseptal canals. 



2. — Portion of the outermost and penultimate con- 

 volutions, laid open by vertical fracture : a, a , 

 e, e, /, /?, y, h, h, as in the last figure ; h', h' , 

 transverse sections of spiral canals ; k, k, spiral 

 lamina, that of the last whorl coalescing with 

 that of the penultimate whorl at k' , k' , and 

 augmenting its thickness ; 75 diameters. 



3. — Portion of the marginal cord, a" , a", as seen in 

 a section passing through the median plane, 

 showing the fusiform spaces formed by the 

 inosculation of the canals ; 120 diameters. 



4. — Portion of a section through the median plane, 

 showing the connection of one of the inter- 

 septal canals g, lying in the midst of non- 

 tubular substance, with the reticulation of 

 the marginal cord a" a'" ; 100 diameters. 



3. — Exterior of the marginal cord, showing at a, a 

 its furrowed surface, a a the transverse section 

 of its canal system, and at li h' the trans- 

 verse sections of the spiral canals which lie 

 just at its junction with the ordinary tu- 



FlG. 



6.- 



bular substance k, k, forming the lateral walls 

 of the chambers. 



-Section through the median plane of a very 

 young OpercuUna, showing the origin of the 

 spire from a globular primordial chamber, and 

 showing the presence of the spiral canal and 

 of its extensions even in the very first convolu- 

 tions ; 75 diameters. 



-Portion of a vertical tangential section passing 

 near the margin of the innermost convolution 

 of a young Operculina, and afl'ording a most 

 advantageous view of its canal-system ; «', a , 

 marginal cord of the inner convolution ; g, g, 

 interseptal canals, springing from h, h, the 

 spiral canals ; i, i, cones of non-tubular sub- 

 stance which take the place of the ordinary 

 tubular substance of the spiral lamina, k, k, at 

 the parts where this is joined by the septa ; 

 80 diameters. 



8 



9.- 



Portions of the spiral lamina magnified 250 

 diameters, sliowing at a its prismatic appear- 

 ance, with a tubulus in the centre of each 

 prism, and at a the large orifices of the tubuli 

 upon its internal surface. 



•Very thin section of a portion of the spiral 

 lamina, showing the ordinary appearance of its 

 tubuli divided transversely, with a delicate 

 areolation between them ; 100 diameters. 



10. — Vertical section of the inner convolutions, which 

 has happened to pass close to the primordial 

 chamber and the one that springs from it, and 

 which shows at a, a, the two spiral canals 

 running along their exterior ; 80 diameters. 



11. — Vertical section of a small Operculina, showing 

 the general distribution of its interseptal canal- 

 system, and the marked difference in thickness 

 between the spiral lamina of the last and that 

 of the penultimate convolutions ; 40 diameters. 



12. — Section of the spiral lamina of a tuberculated 

 variety, showing the partial deficiency of the 

 tubuli in certain spots, b, b, as well as in the 

 septal bands a a, a' a', with their crowding to- 

 gether in the neighbourhood of these ; 120 

 diameters. 



Internal surface of the spiral lamina of another 

 chamber of the same, showing the uniform 

 distribution of the tubuli, except along the 

 septal bands, and the comparatively large size 

 of their orifices ; 120 diameters. 



1 



