GENUS CORONULA. 399 



of growth. Hence the walls offer a strong contrast in ap- 

 pearance with the six, smooth radii, of equal breadth. The 

 symmetrically hexagonal, or rounded-hexagonal orifice of 

 the shell is closed by a thick, nearly horizontal membrane, 

 supporting, towards the rostral end, the small opercular 

 valves, with a slit, having protuberant lips, in the middle, 

 for the protrusion of the cirri. The opercular membrane is 

 attached all round, but a short distance beneath the summit 

 of the shell. In regard to size, I have seen a specimen 

 of C. diadema two inches in height and two and a half in 

 diameter. 



Structure of the Shell. — The structure at first appears 

 singularly complicated, and quite unlike that of any other 

 Cirripede ; but the whole results simply from the folding 

 of the very thin walls, which in all essential respects are 

 constructed like those in Balanus, In a young specimen 

 of C. balanaris, having the orifice of the shell T 2 s ths of an 

 inch in diameter, I found the upper part of each compart- 

 ment only slightly sinuous, not more so than is common in 

 many varieties of Balanus, but more symmetrical, for 

 each compartment had three slight furrows, making, for 

 the whole shell, eighteen furrows. The ensuing changes 

 during the growth of the shell will be best understood by 

 looking at the diagrams a, b, c, in fig. 10, PI. 1 5, which are 

 supposed to represent the basal margins of the walls of a 

 single compartment : {a) shows the simply sinuous wall of 

 the young shell. As the shell grows, the furrows rapidly 

 grow deeper and deeper [b), and wider and wider; at the 

 same time, the folds or ridges between the furrows gradually 

 become drawn out at their ends into transverse loops (<?), 

 the extremities of which ultimately become closely pressed 

 together — the furrows being thus converted into cavities, 

 extending from the top to the base of the compartments. 

 This structure in the mature shell will be best understood 

 by looking at the transverse or horizontal section (PL 16, 

 fig. 7) of the rostral end of the shell of C. diadema, in which 

 species the folding of the walls is simpler than in C. balce- 

 naris or C. reginm : the walls (e) are represented by a 

 double line connected by little cross lines, — (h) being one 

 of the transverse loops at the outer end of one of the folds, 



