178 BALANID^E. 



the latter form being generally assumed when specimens are 

 crowded together; but some species, as B. balanoides, cre- 

 natu8 y and J avis, seem more subject than others to be thus 

 affected. The colour is either white, generally tinted by the 

 yellowish or brownish epidermis, or any colour intermediate 

 between bright pink and rich blue, purple being the pre- 

 vailing tint. The persistence of the so-called epidermis is 

 very different in different species, being even sometimes 

 highly variable in the same species. The surface is either 

 smooth or more commonly folded longitudinally, or sharply 

 ribbed. The orifice differs in form from diamond-shape 

 to trigonal; the carinal end, owing to the shape of the 

 carina, being always sharper or narrower than the rostral end. 

 The size of the orifice, in proportion to the shell, varies accord- 

 ingly as the latter is more or less conical or cylindrical. The 

 orifice is either entire or more or less deeply toothed, in pro- 

 portion to the degree of obliquity of the summits of the radii 

 and alae. The radii almost always have smoother surfaces than 

 the parietes. In some few species the radii are not developed, 

 the sutures being marked only by fissure-like lines; in others 

 they are very narrow, and in this case their upper margins 

 are generally rounded and smooth, instead of being straight 

 and jagged. The carino-lateral compartments are usually 

 much narrower than the lateral compartments, occasionally 

 in an extreme degree, as in B. allium. The shell is 

 generally strong, sometimes to a wonderful degree ; but 

 the strength and thickness vary in the individuals of some 

 of the species. By the action of hot caustic potash, the com- 

 partments in several species, such as B. Humeri and crenatus, 

 separate on a touch ; in others, they adhere so strongly as 

 to prove that the sutures must be calcified together. In 

 this genus we have the largest known sessile cirripede, 

 viz., the B. psittacus, and on the other hand many small 

 species ; but it is very difficult, except in well-known species, 

 to ascertain the average or even the maximum dimen- 

 sions. 



Scutum. — This valve is almost triangular, with the basi- 

 tergal corner more or less rounded off. The prominent 

 lines of growth are sometimes crossed by longitudinal striae. 

 Internally, the articular ridge projects to a very different 



