4 INTRODUCTION. 



variable in the fossil B. concavus and tintinnabulum; the size and form of the little cavity 

 for the lateral depressor muscle varies in many species ; so does the exact shape and degree 

 of prominence of the articular ridge. There is one character in the terga, which at first 

 Avould be thought very useful, namely, whether an open longitudinal furrow, or a closed 

 fissure runs down outside the valve from the apex to the spur ; but it is found that the furrow 

 almost always gradually closes up during growth ; and as a consequence of this, the width 

 of the spur compared to that of the whole valve, as well as its distance from the basi-scutal 

 angle, and the form of its basal extremity, all vary in some degree. The length of the 

 spur sometimes varies considerably, as in B. concavus. The summits of the radii are apt 

 to be oblique in the young of some species, whereas they are generally quite square in the 

 old of the same. In some species 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, and to the consequences which result from this. 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 some 

 species. In all the species, the carino-lateral compartments, in early age, are very 

 narrow in proportion to the width of the lateral compartments ; and in all, at this early 

 period, the operculum is large in proportion to the whole shell. 



Finally, I must state my deliberate conviction that Sessile Cirripedes can very seldom 

 be satisfactorily identified in a fossil condition, without an examination of the opercular 

 valves : hence when these have not been discovered, I have resolved, with some rare 

 exceptions, not to attach a specific name to a shell without its operculum; for thus, I 

 believe, I should add to the number of useless synonyms, which, as we shall immediately 

 see, already exist. Nothing, indeed, could have been easier than to have affixed names to 

 many groups of specimens, having different aspects, but to feel sure that these were really 

 distinct species requires better evidence than can be afforded by the shell, without the 

 opercular valves. When the specimens are much fossilised, it is, indeed, difficult to make 

 out the primary points of structure in the genus Balanus — namely, whether the parietes, 

 radii, and basis are porose : to do this it is sometimes necessary to rub down, polish, and 

 carefully examine, a transverse section of a piece of the shell. 



The ancient history of the Balanida? is a brief one. No Secondary species has hitherto 

 been discovered; in my former monograph on the fossil Lepadidae 1 I have shown that 

 the negative evidence in this case is of considerable value, and consequently that there is 

 much reason to doubt whether any member of the family did exist before the eocene period. 



1 Since the note to page 5 of that work was written, I have been informed that the so-called cretaceous 

 Tubicinella maxima is not a Cirripede. 



