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 ; b b one of the spiral canals ; c, c, septal bands of transparent shell-sub- 

 stance ; d, tubular shell-substance of spiral 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 ; c, septal band ; d, 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 Operculina, traversing its median plane except in its central portion, 

 and showing, especially at a and i, great irregularities in the disposition of 

 its chambers : 12 diam. 



Fig. 4. Portion of a specimen of Operculina, 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 b ; the 

 marginal cord appears to have been continued from c to a along the line of 

 fracture, and thence to have grown onwards to b : 50 diam. 



Fig. 5. Vertical section of Amphistegina Cumingii, showing its conformity in all essen- 

 tial respects to the type of Operculina, and the differences produced by the 

 smaller size of its chambers in proportion to the thickness of its spiral 

 lamina: 50 diam. 



Fig. 6. Portion of section of Amphistegina Oumingii, 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. 



