A 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 
would 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 résolved, 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 Balanidz is a brief one. No Secondary species has hitherto 
been discovered; in my former monograph on the fossil Lepadide’* 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. 
