EXPLANATION OF THE PLATES. 
41 
PLATE VI. 
Fig. 1. Section of a portion of the outer whorl of Operculina^ parallel to the surface, 
showing a marked irregularity in the disposition of the septa : — a a, marginal 
cord ; h h one of the spiral canals ; c, c, septal bands of transparent shell-sub- 
stance ; d, tubular shell-substance of sphal lamina forming the walls of the 
chambers ; e, e, prolongations of interseptal canals in spiral lamina : — lOOdiam. 
Fig. 2. Similar section from another specimen, showing an appearance of reticulation in 
the spiral lamina : — a a, marginal cord ; r?, septal band ; one of the inter- 
septal canals communicating with canal-system of marginal cord ; e, prolon- 
gation of interseptal canals in spiral lamina : — 100 diam. 
Fig. 3. Section of Ojperculina, traversing its median plane except in its central portion, 
and showing, especially at a and 5, great irregularities in the disposition of 
its chambers: — 12 diam. 
Fig. 4. Portion of a specimen of Opey'culina, which seems to have undergone fracture at 
an early age, and to have thenceforth grown in a reversed direction, as will 
be seen on comparing the direction of the septa in the whorls a and h ; the 
marginal cord appears to have been continued from c to a along the line of 
fracture, and thence to have grown onwards to d : — 50 diam. 
Fig. 5. Vertical section of AmpMstegina Cumingi% showing its conformity in all essen- 
tial respects to the type of Operculina, and the differences produced by tha 
smaller size of its chambers in proportion to the thickness of its spiral 
lamina: — 50 diam. 
Fig. 6. Portion of section of AmpMstegina Cumingii, traversing its outer whorls in the 
median plane, but passing along the surface of its inner convolutions, so as 
to bring into view the alar prolongations of the septa in their passage towards 
the centre, and the occasional inosculation of their canal-systems by loops in 
that region : — 50 diam. 
MDCCCLIX. 
a 
