188 BALANIDJS. 



siderably, and are affected by the general shape of the 

 shell. Unfortunately the differences are not very great 

 between the scuta of the different species. The cause of 

 the opercular valves offering more useful characters, as 

 far as outline is concerned, than do the walls of the shell, 

 is no doubt due to their being almost independent of 

 any influence from the nature of the surface of attachment. 

 Even the ridges and depressions on the under side of the 

 scuta, which are in direct connection with the muscles and 

 soft parts of the animal, vary to a certain extent : thus the 

 length and prominence of the adductor ridge is decidedly 

 variable in B. concavus and tintinnabulum, and in a less 

 degree in B. lavis ; 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 arti- 

 cular 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 

 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 com- 

 pared 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 some- 

 times varies considerably, as in B. concavus and amjjlii- 

 trite. The parts of the mouth are only occasionally ser- 

 viceable ; for the teeth on the labrum, and the state of the 

 lower teeth on the mandibles, and the presence of a step- 

 formed projection at the lower angle of the maxillae, are 

 all often variable. The relative lengths of the two rami 

 of the first pair of cirri, the degree of protuberance of the 

 segments, and the number of pairs of spines on the seg- 

 ments of the posterior pairs of cirri, are sometimes useful; 

 but the relative lengths of the cirri, and more especially the 

 numbers of pairs of spines on the posterior cirri, are apt 

 to vary. Finally, I must express my deliberate opinion, 

 that every part and organ, internal and external, in Cirri- 

 pedes, is liable to some amount of variation in some of the 

 species. 



I must now point out the principal changes which super- 



