338 BALAN1D.E. 



rally covered by a brownish epidermis, thickly clothed with little 

 spines ; the orifice is neatly diamond-shaped ; the apices of the oper- 

 cular valves meet at a common point : these specimens are almost 

 always smaller and younger than the granulated specimens. Altoge- 

 ther the specimens in the two opposite states have, in their external 

 appearance, nothing in common, and no one, without careful examina- 

 tion, would ever suspect that they were specifically identical ; this, 

 however, was proved by the intermediate forms, and, in one instance, 

 three of the compartments had their surfaces granulated, and were en- 

 tirely destitute of the radii, whilst the fourth by some chance had been 

 preserved from corrosion, was longitudinally ribbed, and had its epi- 

 dermis-covered radius fully developed. The difference in the appear- 

 ance of the opercular valves, in the two states, is simply owing to the 

 degradation of their upper parts in the granulated specimens. 



The basal diameter of the largest specimen was one inch, but the 

 height only *35 of an inch. 



Scuta, transversely elongated, so that the basal margin is nearly 

 twice as long as the tergal margin : articular ridge very little prominent; 

 articular furrow wide but shallow; adductor ridge very blunt, slightly 

 prominent, sometimes almost absent, almost parallel to the basal mar- 

 gin : there are no distinct crests for the rostral or lateral depressores 

 muscles, but some small irregular pits for the latter. In one young 

 specimen, the lines of growth were crenated, showing a tendency in 

 the valve to become longitudinally striated, as in the allied T. costata. 

 In some young and immature specimens, the basal margin was deeply 

 sinuous. 



Terga, small in area, not above half that of the scuta: spur extremely 

 short, broad, placed close to the basi-scutal angle of the valve, so that 

 there is no basal margin on that side of the spur. The lower end and 

 sides of the spur form one uniform curve. Articular ridge barely deve- 

 loped. Crests for the tergal depressores sharp and prominent. 



Structure of the Shell and Radii. — The walls are very thick, and the 

 parietal tubes small and numerous; there are sometimes from twelve to 

 fifteen rows of tubes in the thickness of the wall. The tubes in their 

 whole upper part are filled up solidly ; and, as we have seen, are often 

 exposed by disintegration. In very young specimens, of that size in 

 which in T. porosa there would be only a single row of parietal tubes, 

 there were here two or three rows. The development of the radii, as 

 we have seen, is very capricious ; the sutures even sometimes being 

 lost. The radii, when developed, are broad, square on the summit, 

 and covered by brownish hirsute epidermis: internally they are formed 

 of tubes like those forming the parietes ; in this respect differing from 

 all the species except the following, T. costata. The tubes in the 

 radii run obliquely down towards the basis ; instead of in a transverse 

 line, directly towards the opposite compartment, as might have been ex- 

 pected from the structure of the radii in Balanus. The aire have their 

 edges finely crenated. The sheath in all the specimens which I have 

 observed is colourless ; its lower edge is not free. The corium entering 

 the parietal tubes, and lining the opercular valves, the mouth, and the 

 anterior cirri, is generally of an extremely dark purple colour. 



