186 balanidjE. 



full-grown specimen ; and I believe, as long as the individual 

 lives, so long does it go on moulting and growing. Colour is 

 of very considerable service ; though the precise tint varies 

 greatly in almost every species ; and what is a far more 

 serious evil, the majority of the species have their white or 

 nearly white varieties, the latter sometimes as numerous as 

 the coloured ones: in B. per for at us, Icevis, Jlosculus, awpJii- 

 trite, and in several other species, the common white varieties 

 are eminently deceptive. 



Besides the slight variation in the obliquity of the summits 

 of the radii and alse, dependent on the more or less cylin- 

 drical form of the shell, in some species, as in B.tintinnabulum, 

 amphitrite, improvisus, trigonus, and porcatus, their obliquity 

 also varies occasionally from unknown causes, and thus great- 

 ly affects the general appearance of the shell. In some few 

 species, as in B. perforatum, the radii are often either not at all 

 developed, or are of very variable width ; in others, when the 

 shell has become cylindrical, or when very old, the radii cease 

 to grow, and from the disintegration of the whole upper part 

 of the shell, with the continued growth of the lower part, 

 the radii at last come to exist as mere fissures : I have seen 

 instances of this in B.psittacus, nigrescens, and porcatus. 

 Nevertheless, the obliquity of the npper margin, and the 

 breadth of the radii are useful characters ; and still more 

 useful is the fact whether the upper margins are smooth 

 and arched, or straight and jagged. The fact of the terga 

 being more or less beaked is useful : as is, likewise, the 

 presence of stria?, or furrows, or rows of pits, radiating 

 from the apices of the scuta; but to ascertain the pre- 

 sence of these marks, it is almost invariably necessary to 

 take out the scuta, clean, and examine them with a lens • 

 these ridges and furrows, moreover, in some species, as is 

 strikingly the case with B. tintinnabulum, and in less degree 

 with B. trigonus, Icevis, and co?icavus, appear and disappear, 

 and vary without any apparent cause. 



Now if we reflect that form, size, state and nature of 

 the surface, presence of epidermis, relative size of the 

 orifice, presence of longitudinal ribs, tint, and often the 

 existence of any colour, are all highly variable in most of 

 the species ; and that the obliquity of the summits of the 



