GENUS TKTRACLITA. 327 



quite as much as in Balanus, for we very rarely here see 

 cylindrical varieties; but in the same species, we have 

 extremely depressed, steeply conical, and convex forms ; 

 the orifice varies in relative size ; the state of the surface, 

 whether ribbed or smooth, whether well preserved or cor- 

 roded, the upfilled parietal tubes being thus exposed, varies 

 more than in Balanus. The colour also varies ; specimens of 

 T.porosa (PL 10, fig. 1 a to \f) being dark purple, or even 

 inky black, white, pale pinkish-purple, and green : as far as 

 I have seen, the pinkish varieties of T.porosa are confined to 

 the eastern half of the world. Even the ridges and crests on 

 the under side of the scutum vary in some degree ; and the 

 tergum in T porosa varies considerably, and in some of the 

 other species, slightly, in general shape : I believe that the 

 tergum becomes narrow and elongated, when the shell is 

 steeply conical, with the orifice of small diameter. After 

 having spent several days in disarticulating and closely ex- 

 amining the many specimens of T.porosa in my possession, 

 I concluded that its varieties formed at least four species, 

 these being in external appearance extremely distinct ; but 

 ultimately the many intermediate forms compelled me to 

 unite all into a single species. Again, I may instance the 

 conical, ribbed variety of T. purpurascens (PL 11, fig. 1 b), 

 with the outer lamina of the shell preserved and with the 

 radii widely developed, as having not the smallest resem- 

 blance to the other common depressed variety (fig. 1 a), 

 having a granulated surface, produced by the exposed tips of 

 the upfilled parietal tubes, and without a trace of the radii 

 or even of the sutures : I mav add that, according to the 

 characters used by some authors, these two varieties would 

 be classed in two separate genera. 



To distinguish the species of this genus, the chief reliance 

 must be placed (as in the case of most other sessile cirri- 

 pedes) on the general outline of the opercular valves, and 

 on the ridges and crests on their under sides : the internal 

 structure, however, of the radii, and in a lesser degree of 

 the parietes, must not be overlooked. I have not found the 

 parts of the mouth, or the differences in the cirri, of much 

 service; and it will be shown under T porosa, that the 



