2 INTRODUCTION. 



history of Cirripedes has been neglected ; and this remark is eminently applicable to the 

 Balanidae, or Sessile Cirripedes. Even the British recent species have not been well made 

 out, and as for the fossil species, scarcely anything has been done, besides the publication of 

 some figures, in very few instances accompanied by the details which are absolutely neces- 

 sary for the identification of the species. 



Owing to the great variation in external characters, to which almost all the species are 

 subject, and likewise in the case of the principal genus, Balanus, to its being a very natural 

 genus, that is, to the species following each other in close order, it is not easy to 

 exaggerate the difficulty of identifying the species, except by a deliberate examination of 

 the internal and external structure of each individual specimen. Every one who has 

 collected Sessile Cirripedes must have perceived to what an extent their shape depends on 

 their position and grouping. The surface of attachment has a great effect on that of the 

 shell ; for as the walls are added to at their bases, every portion has at one time been in 

 close contact with the supporting surface ; thus I have seen a strongly-ribbed species 

 {B. porcatus) and a nearly smooth species (B. crenatus) closely resembling each other and 

 both having a peculiar appearance, owing to their having been attached to a pecten. 

 Dr. Gray has pointed out to me specimens of the recent B. patellaris, curiously pitted like 

 the wood to which they had adhered ; and numberless other instances might be added. 

 Quite independently of the effect produced by the surface of attachment, the degree to 

 which the longitudinal folds and ribs are developed on the parietes, is variable in most of 

 the species, as in B. tintinnabulum and even in B. porcatus; the presence or entire 

 absence of these ribs often surprisingly alters the whole aspect of the shell. The persistence 

 of the so-called epidermis is in some degree variable, though this is of little importance in 

 regard to fossil specimens. Again, some species in certain localities are all subject to the 

 disintegration of the entire outer lamina of the walls; and in such cases (as with 

 B. per/oratus) there is not the smallest resemblance between the corroded and perfect 

 specimens. The size of the orifice, and consequently of the operculum, compared with the 

 shell, varies accordingly as the shell is more or less conical or cylindrical ; in the 

 latter case, the summits of the radii are generally more oblique and the orifice consequently 

 more deeply toothed than in the more conical varieties. Size is a serviceable character in some 

 cases, but very many specimens are required to ascertain the average or maximum size 

 of each species, for there is no method of distinguishing a half-grown from a full-grown 

 specimen ; and I believe, as long as the individual lives, so long does it go on growing. 

 Colour is of very considerable service; but the majority of the species have their 

 white or nearly white varieties, the latter being sometimes as numerous as the coloured 

 ones. 



Besides the slight variation in the obliquity of the summits of the radii and ate, de- 

 pendent on the more or less cylindrical form of the shell, in some species, as in B. 

 tintinnabulum and porcatus, their obliquity also varies occasionally from unknown causes, 



