84 BALANIDiE. 



the whole ramus may be said to be antenniformed, and I 

 believe acts as an organ of touch : the relative number of 

 the segments, I may add, in the two rami and the arrange- 

 ment of their spines varies greatly in this species. In two 

 other species of the same genus Chthamalus, we have 

 occasionally the anterior ramus in some degree antenni- 

 formed, so that this whole structure is variable. In the 

 allied Cliamcesipho columna, it is the posterior or inner 

 ramus which is antenniformed, but this peculiar develop- 

 ment is more plainly marked in the case of the second pair 

 of cirri than in that of the third pair. In Tetraclita porosa 

 it is, also, the posterior ramus of the third pair which is 

 antenniformed ; in this third pair, and indeed in the other 

 cirri, the relative numbers of the segments vary extremely. 

 A similar structure in the third pair, but in a lesser and 

 variable degree, may be observed in some of the other 

 species of Tetraclita. In Balanus vestitus, also, we have, in 

 the third pair, an analogous structure. It is scarcely possible 

 to believe that the circumstance of the second pair of legs, 

 which answer to the third pair of cirri, being antenniformed 

 in certain decapod Crustaceans, is an accidental coincidence ; 

 it must be owing to some special affinity in the two groups. 



In Chelonobia, the third pair of cirri is of unusual 

 length compared with the second pair, but does not other- 

 wise differ from the type of its sub-family : in Coronula and 

 its allies, on the other hand, the third pair is very short and 

 broad, as may be seen (PI. 29, fig. 5) in Xenobalanus: 

 in this latter genus, the front surfaces of the segments of 

 the pedicels (fig. 6) of the posterior cirri, are extremely 

 protuberant, almost as in Scalpellum vulyare. 



The last peculiarity in the cirri at all worth mentioning, 

 is in the sub-genns Acasta, in which, differently from in all 

 other known Cirripedes, the anterior ramus of the fourth 

 pair does not absolutely resemble the rami of the fifth and 

 sixth pairs ; in most of the species, the spines on this anterior 

 ramus are more crowded together, are larger, and are mingled 

 with some short thick points ; and the spines in the dorsal 

 tufts are also longer than in the two posterior pairs of 

 cirri; but in A. sulcata (PI. 29, fig. 2), and in a lesser 

 degree in A. cyathm and A. p?trpurala, the front margins of 



