GENUS TKTRACLITA. 323 



instead of crests, minute pits for the attachment. These 

 crests vary much in prominence in the same species. 



Terga. — These valves preseut no essential differences 

 from those of Balanus ; they are sometimes beaked, and the 

 beak is hollow and occupied by a thread of corium, as in that 

 genus. The external surface of the valve is often depressed 

 in the line of the spur, but there is never a longitudinal 

 furrow with the edges folded in, as in Balanus. The spur is 

 very short in T. purpurascens. In T. radiata, the articular 

 ridge is remarkably prominent. The crests for the depressor 

 muscles are well developed in all the species. The shape of 

 the terga is variable in nearly all the species, and greatly so 

 in T. porosa. 



Compartments. — Owing to there being only four compart- 

 ments, the lateral pair are large ; the size of the carina rela- 

 tively to the rostrum varies, according as its aloe have been 

 added to during diametric growth. The walls are very thick, 

 and are composed of numerous tubes, in some species as 

 many as fourteen or fifteen rows being exposed on the basal 

 margin (PL 10, fig. \g). The tubes are generally angular, 

 and slightly elongated in the ray of the circular shell ; 

 sometimes they are nearly circular and small. New tubes 

 are formed only at the basal edge of the outer, lamina, by the 

 bifurcation of the septa which form the tubes. In very young 

 specimens there is only a single row of tubes ; and in T. rosea 

 this holds good throughout life : in this species (fig. 3 d) the 

 tubes, in the single row, are large and quadrangular, and the 

 outer lamina of shell is strengthened by numerous, small, in- 

 ternal, longitudinal plates. I believe the branching septa, 

 which separate and form the parietal tubes, correspond with 

 the longitudinal septa in the more simple walls of Balanus. 

 The tubes become solidly filled up, in their upper parts, 

 with hard, and generally coloured shelly matter. The degree 

 to which they are filled up differs in the different species • 

 the external side of each tube is always first thus coated. 

 The thin outer lamina of shell, in several of the species, 

 commonly disintegrates and disappears ; the upfilled parietal 

 tubes being thus exposed. The inner lamina of the walls is 

 generally smooth, but in T. radiata it is longitudinally ribbed, 

 as in most species of Balanus. The sheath is generally dark- 



