GENUS BALANUS. 189 



vene during growth, and which cannot properly be called 

 variations. In the first place, I think, it is scarcely possible 

 to recognise a species when under the ^th of an inch in 

 diameter. In some cases, as in var. tV Orbignii of B. 

 tinUnnabulum, the shell is invariably coloured when old, but 

 quite white when very young. Generally the tints become 

 very much darker with age. Some species, which usually 

 or invariably have, when mature, longitudinally folded walls, 

 as B. Jlosculus and balanoides, are perfectly smooth in 

 youth. The walls in B. eburneus, when young, have white, 

 hyaline, longitudinal lines, and are naked ; whereas, with 

 advancing age, these lines disappear, and the subsequently 

 formed shell becomes covered with membrane. The sum- 

 mits of the radii are apt to be oblique in the young of B. 

 Capensis, psitfacus, and tintinnabulum, whereas they are 

 generally quite square in old specimens. In B. eburneiis, 

 cariosus, and in a lesser degree in B. psittacus, the scuta 

 become longitudinally striated only with age. On the other 

 hand, in very young specimens of B. tintinnabulum, the 

 scuta sometimes are deeply impressed by little pits placed 

 in rows. I have already alluded to the longitudinal furrow 

 on the tergum so entirely changing its character, owing to 

 the edges becoming, during growth, folded inwards ; this 

 likewise causes a decrease in the proportional breadth of 

 the spur. In old specimens of B. Jlosculus, var. sordidus, 

 the whole tergum is much more elongated than in young 

 specimens. The basal margin of the sheath is hollow 

 beneath in the young of B. cariosus and of some other 

 species, but in the old it is continuous with the inner sur- 

 face of the walls. The inner lamina of the parietes generally 

 loses, to a certain extent, its longitudinally ribbed character 

 in old age. The basis is solid, instead of being porose, in 

 very young specimens of B. improvisus. In all the species, 

 the carino-lateral compartments, in early age, are very 

 narrow in proportion to the width of the lateral compart- 

 ments ; and in all, at this early period, the operculum is 

 large in proportion to the whole shell. The number of 

 spines on the edge of the maxillae, the number of segments 

 in all the cirri, and the number of spines on each seg- 

 ment, are few in early youth, and go on increasing with 



