GENUS COKONULA. 405 



Radii. — The radii are very wide in C. diadem a. In all 

 the species their summits are square or parallel to the basis. 

 Their internal structure is remarkable: as the walls in this 

 genus are extremely thin, so are the proper radii, for in fact 

 they consist in this and all cases, as we know, of one margin 

 of the wall modified by its lateral growth against the opposed 

 compartment. But as the radii in Coronula are not plicated, 

 like the walls, the shell would have been excessively weak 

 along the six lines of suture, had not the radii been 

 strengthened by numerous sinuous plates, springing from 

 the inner lamina of the proper radius, and running down- 

 wards, attached to the folded wall of the compartment to 

 which the radius belongs, and with their free edges pressed 

 against the folded wall of the opposed compartment. These 

 plates give out short transverse denticuli, making altogether 

 a beautiful structure, as is best seen in PL 16, fig. 3, but 

 also in fig. 2 and 4, and dm fig. 1. In the section, fig. 7, 

 the proper radius (d) is seen to be continuous with the wall 

 (e), and to be very thin, in fact forming but a small portion 

 of the compound radius : it is formed of an outer and inner 

 approximate lamina, separated by septa, which are nearly 

 horizontal, and which consequently cannot be shown in 

 the transverse section fi°\ 7. The outer lamina of the 

 radius is imperfect, or does not reach quite to the suture, 

 leaving the septa a little exposed (imperfectly shown in 

 PL 16, fig. 3), exactly as is the case with the outer lamina of 

 the parietes, at the basal margin of the shell. In all common 

 Balanidse, a ribbon of corium runs up each of the six sutures, 

 and sends in fine threads between the septa of the radii, 

 but here a thread of corium runs up a minute, cylindrical 

 pore, situated on the line of junction between the radius and 

 the wall whence it arises ; and from this longitudinal thread 

 the finer threads spring which pass between the horizontal 

 septa of the proper radius : this cylindrical pore is rather 

 large in C. balcsnaris, but excessively small in C. diaderaa 

 (see a black dot [d') in section 7), and is solidly filled up in 

 the upper part of the shell. The plates (fig. 3) which 

 run down from the inner lamina of the proper radius, and 

 form the greater part of its thickness, are occupied by 

 fringes of corium, extending up from a ribbon of corium, 



