328 BALANIDJE. 



relative numbers of the segments in the several cirri, and 

 even their shape, is extraordinarily variable. 



Affinities. — All the species are pretty closely allied, and 

 there is no ground for making any sectional division of the 

 eight species, more especially not on the ground whether or 

 no the radii are developed. Tetraclita purpurascens, taking 

 into account all the characters, including the animal's 

 body, is perhaps the most distinct species in the genus, but 

 it is closely allied to T. coslata. In the well-marked 

 character of the parietes being formed of a single row of 

 large tubes, T. rosea differs from all the other species. The 

 genus is closely allied to Balanus ; I can point out no 

 difference in the animal's body, nor any constant difference 

 in the opercular valves. The ridges and crests on the under 

 sides of the scuta are more prominent in Tetraclita ; and 

 all the species, excepting two, have crests (though sometimes 

 very slight) for the attachment of the rostral depressor 

 muscle, and these occur only in two species of Balanus : 

 crests, also, for the attachment of the lateral depressor 

 muscle are common in Tetraclita, but rare in Balanus. The 

 main character, however, of the genus, as compared with 

 Balanus, is derived from the existence of only four compart- 

 ments, and in a lesser degree from the several rows of 

 parietal pores; but, as just stated, T. rosea has only a single 

 row, and, on the other hand, in Balanus crenatus, there is 

 a slight tendency exhibited, in the dividing of the longi- 

 tudinal septa near the outer lamina, to form a second row of 

 parietal pores; and in B. cariosus, moreover, we actually 

 have two or three rows of irregular and very short pores. 

 The thin, yet solid calcareous basis which occurs in all the 

 species of Tetraclita, excepting T. pu?purasce?is, resembles 

 the basis in Balanus Jloscidus and imper ator, but I suspect 

 that the structure of the cement-ducts is different in the 

 two genera. 



