324 BALAN1DJB. 



coloured ; its lower edge does not project or overhang the 

 inner lamina, as is usual in Balanus, excepting in T. serrata, 

 and in some few varieties of T.porosa. 



The Radii, when developed, are either narrow or 

 broad, with their summits either oblique or extending in 

 a straight line from the top of one compartment to that 

 of another. In T. serrata, I have not seen a single speci- 

 men with the radii developed ; in T. porosa, they are very 

 seldom developed, and then, apparently, only in quite young 

 specimens, in which they are narrow; in T. purpurascens, 

 they seem to be about as often developed as not, and when 

 present they are broad ; in T. costata, cceridescens, and 

 radiate?, they are always largely developed. In some spe- 

 cimens of the species, in which the radii are not developed, 

 even the sutures do not reach the external surface ; the outer 

 lamina of the parietes being continuous all round, so that 

 the shell seems formed of a single piece. Even in such 

 specimens, the four compartments, viewed from within, can 

 be seen to be distinct ; and the sutures can generally be 

 traced across the whole thickness of the parietal tubes ; in 

 this latter case, when the sutures are broken open, the radii 

 are seen to be represented (PL ] 0, 1 h) by a few small 

 sinuous ridges. Owing to the disintegration of the upper 

 and outer part of the shell, and the consequent exposure of 

 parts of the sheath and alae, the radii sometimes appear as if 

 developed, when such is not the case. With respect to the 

 internal structure of the radii, they are formed, in T. purpn- 

 rascens and costata, of tubes, like those of the parietes, and 

 therefore according to the normal plan ; whilst in the other 

 species they are formed by longitudinal sinuous ridges, 

 sending out on each side irregular denticuli ; and the inter- 

 spaces between the ridges are filled up solidly during the 

 growth of the radii, in all the species, except in T. radiata, in 

 which they are left to a considerable extent open. These sinu- 

 ous ridges, with their denticuli, homologically represent the 

 branching septa which form the parietal tubes. The edges of 

 the alie are crenated in all the species, except in T. costata. 



Diametric growth. — AVhen first examining groups of T. 

 porosa, in none of which the radii had been developed, and 

 in which, consequently, the shell could not have grown in 



