GENUS C0R0NULA. 401 



transverse loops of the folded walls are plainly shown. Tt 

 is the more or less rounded surfaces of the transverse loops 

 which give to the external surface of the parietes its longi- 

 tudinally ribbed structure : the ribs are plainest in C. 

 diadema, fig. 3, PI. 15. The shell, in fact, as seen externally, 

 consists of but an extremely small portion of the external 

 surface of the whole length of wall, being exclusively formed 

 of the transversely looped ends of the radiating folds, 

 together with the radii. Owing to the ends of the trans- 

 verse loops being so closely pressed together, the furrows 

 are practically converted (as already remarked) into cavities, 

 open only on the under side of the shell, and extending from 

 the oblique bases of the compartments up to their apices ; 

 and these are invariably filled by the black epidermis of the 

 Whale. Owing to this circumstance, the skin of the Whale 

 has been mistaken by some authors for parts of the Cirri- 

 pede ! In C. diadema, in which the summit of the shell 

 is often a little disintegrated, the whale's skin is often there 

 exposed, forming three black spots at the top of each com- 

 partment. It should always be remembered that these 

 flattened and deep cavities are furrows, which homologicaUy 

 ought to be open in longitudinal lines along the external 

 surface of the shell, from the top to the bottom. 



As the shell increases in diameter, each of the original 

 eighteen transverse loops, forming the exterior surface of 

 the shell, increases in breadth ; and they w r ould have had to 

 increase extremely, had not some of the transverse loops 

 become, during growth, divided into two or three new trans- 

 verse loops, in a manner strictly analogous with the first 

 formation of the eighteen folds in the young shell. In 

 PL 15, fig. 7, 8, 9, we see how one of the circumferential 

 transverse loops, by the formation of a medial furrow, or 

 rather bay, becomes developed into two transverse loops ; 

 and it is rather important to observe that three new loops 

 might equally well have been contemporaneously formed. By 

 the repeated formation of new circumferential loops and the 

 consequent formation of new folds, the wall of the shell, when 

 old, especially in C.balcenaris, becomes folded in a wonderfully 

 complicated manner, as maybe seen in PI. 15, fig. 5, which 

 is an exact tracing of the extreme basal edge of the wall of 



26 



