356 balanid^:. 



colour is white, or pinkish-purple. Most of the species are 

 small, but I have seen specimens of P '. grande three quarters 

 of an inch in diameter in the longer axis, and, including the 

 deep, almost tubular, basis, more than three inches in length 

 or depth. 



Opercular Valves. — In three species, viz., P. conjugation, 

 (PI. 12, fig. 1 c), grande, and wont i cut arice, the scuta and 

 terga, on each side, are calcified together so perfectly, that 

 there is no trace of a suture or line of junction : in P. mil- 

 leporae, these valves are generally slightly calcified together, 

 but with the suture distinct. The Scuta differ so much in 

 shape in the different species, that little can be said of them 

 in common : in P. Anglicnm and Stokesii they resemble 

 those of Balanus ; but in the other species they are much 

 more elongated than is usual, and this is carried to an ex- 

 treme in the last four species ; this elongation is due to a great 

 increase in breadth, as may be inferred from the position 

 of the apex of the valve, and from the direction of the lines of 

 growth. But the two most remarkable characters are, first, — 

 the extraordinary development of the adductor ridge, so that, 

 in P. conjugation, cancellation (PL 12, fig. 5 c), grande, and 

 crenatmn, it extends considerably beneath the basal margin, 

 being produced, in the first two species, at the rostral angle, 

 into a point ; at the tergal end of the valve, the adductor 

 ridge, when thus much developed, blends into the articular 

 ridge. The second very remarkable character is the addition 

 of a special ledge along the occludent margin of the scutum, 

 and along the carinal margin of the tergum, which 1 will call 

 the occludent ledges (limbics occludens), and which serve to 

 close the orifice leading into the sack. The occludent ledge 

 is small in P. grande, and is clothed with thick yellow spines, 

 giving it a brush-like appearance : in P. crenatum and 

 dentation it is largely developed, the ledge on the scutum 

 being articulated with that on the tergum, as shown in 

 PL 13, fig. 4 a, 4b, — the ledges being here and elsewhere 

 marked by little bristly points. In P. monticularice, how- 

 ever, this ledge arrives at its maximum development (PL 13, 

 fig. 5/), the rest of the valve (the scuta and terga being 

 here, as in P. grande, calcified together) being reduced to a 

 mere basal edge or border. Excepting for the adductor 



