GENUS BALANUS. 185 



the genus being a very natural one, that is, to the species 

 following each other in close and natural 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 

 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 longi- 

 tudinal folds and ribs are developed on the parietes, is 

 variable in most of the species, as in B. tinti7inabulum, 

 vestitus, and even in B. porcatus ; the presence or entire 

 absence of these ribs often surprisingly alters the wdiole 

 aspect of the shell. The persistence of the so-called 

 epidermis is in some degree variable ; and in B. Icevis we 

 have groups of specimens absolutely naked, and others 

 uniformly clothed with a brown membrane. Again, some 

 species in certain localities are all subject to the disintegra- 

 tion of the entire outer lamina of the walls ; and in such 

 cases (as with B. perforates) there is not the smallest resem- 

 blance between the corroded and perfect specimens. The 

 size of the orifice, and consequently of the operculum, 

 compared with the shell itself, 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 

 aperture 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 



