404 BALANIDiE. 



otherwise the included threads of corium would have been 

 exposed. Each fresh period of growth, in the case of C. 

 diadema (fig. 6) and regince, and to a certain extent in 

 0. bala?iaris, is marked by little knobs on the longitudinal, 

 slightly prominent, septa, and this prefigures an analogous 

 strongly marked structure in Tubicinella. A fine thread of 

 corium runs up each pore to the summit of the compartment ; 

 for these pores are not, as in Balanus, cut off by transverse 

 calcareous septa, or have their upper ends solidly filled up 

 with shelly matter. As, however, the summit of the shell in 

 Coronula is sometimes disintegrated, the threads of corium 

 within the pores would have been exposed, had not each thread 

 formed for itself, as I suppose, a transverse membranous sep- 

 tum near the summit of the shell ; at least this is the case 

 with the larger pores of the radii. The walls, where closely 

 pressed together in the spoke-like folds, are disunited at the 

 extreme base, but above this they are firmly calcified together. 

 A ribbon of corium runs along the basal edge of each spoke, 

 and sends threads of corium up the parietal pores on each side, 

 and its upper edge serves to deposit calcareous matter (homo- 

 logous with the layers of the sheath) and thus to unite the two 

 walls firmly together. In C. diadema, the walls of the terminal 

 transverse loops are simply calcified together like the spoke- 

 like portions ; but in C. balcenaris the opposite sides of the 

 loops are united by septa (see the transverse section in 

 PI. 15, fig. 2 a), making from five to eleven longitudinal 

 tubes within each transverse loop ; these tubes being larger 

 than the parietal pores. When a piece of the shell is dis- 

 solved in acid, no tubuli can be discovered, which may be 

 accounted for bv the thinness of the walls : nor are there 

 any spines on any of the external membranes. The number 

 of the pores, in the parietes of a moderately-sized specimen 

 of C. balcenaris, I calculated was at 3400, each occupied by 

 a thread of corium springing from the eighteen branched 

 ribbons, diverging from the corium, surrounding the base 

 of the sack. To this number must be added between 300 

 and 400 larger threads of corium running up the tubes in 

 the transverse loops; and no less than about 2300 fringes and 

 threads occupying the pores in the six radii: thus we see that 

 the dermal system in Coronula is wonderfully complicated. 



