BALANUS. 27 



General Appearance. — Shell (fig. 2a) conical or tubulo-conical, often rather globose ; 

 walls frequently thin, either very smooth, or deeply plicated longitudinally : occasionally 

 the same specimen is smooth in the upper part (fig. 2/5), and strongly plicated in the 

 lower. The radii in the large specimens are wide, and with their upper margins only 

 slightly oblique ; in the smaller they are narrower, and much more oblique ; but in each 

 case their upper margins are smooth and slightly bowed. Colour apparently originally 

 nearly white, but with the alse generally, in the smaller specimens, clouded with a dark 

 tint : the radii are usually striped feebly in longitudinal lines. Basal diameter of largest 

 specimen *8 of an inch ; but this seems to have been an unusual size. 



Scuta : (fig. 2e) narrow, with the basal margin forming an unusually small angle with 

 the occludent margin ; surface slightly convex, with lines of growth approximate, mode- 

 rately prominent ; on the tergal half of the valve, two distinct rather broad furrows, with 

 sometimes a third, and even a fourth, nearer to the occludent margin, extend from the 

 apex down the valve, and give it a very peculiar appearance : the furrows near the tergal 

 margin are the deepest. Internally (fig. 2g), the upper part of the valve is roughened 

 with small points : the articular furrow is unusually wide : the articular ridge is very pro- 

 minent and but little reflexed, with the lower end almost abruptly cut off: the adductor 

 ridge is prominent, but short : there are small deepish pits for the rostral and lateral 

 depressores. 



Terga (fig. 2/), broad, flat, with a slight narrow prominent rim along the scutal 

 margin, which margin is slightly bowed. The basal margin on the carinal side of the spur 

 slopes so gradually towards the spur, that the latter is barely distinct, and is very short, not 

 depending nearly half its own width beneath the basi-scutal angle : the spur, also, is broad, 

 namely, measured across the upper part, as broad as half the valve ; its basal end is obliquely 

 rounded off on the carinal side ; it is placed close to basi-scutal angle. The carinal margin 

 of the valve is just perceptibly bowed, and is formed by rectangularly upturned lines of 

 growth. Internally (fig. 2h), the upper part of the valve is rough ; the articular ridge is 

 prominent ; the crests for the tergal depressores muscles are moderately well-developed. 



Parietes, not porose ; internally, the ribs are smooth, with their basal edges very finely 

 or barely denticulated. The radii (as already stated) are of variable breadth ; they have 

 their upper margins either very slightly or highly oblique, but always smooth and rounded: 

 their sutural edges are quite smooth, or sometimes, with a strong lens, traces of transverse 

 striae, representing septa, can just be discovered. The alee have their upper margins very 

 oblique ; their sutural edges are, in the large specimens, quite smooth ; in the younger 

 ones, plainly crenated ; the recipient furrow being clearly marked by the teeth. Basin 

 plainly porose. 



Varieties. — It is certain (fig. 2b) that there are longitudinally plicated specimens of 

 this species, and that the obliquity of the upper margins of the radii also varies a little ; 

 nevertheless some of the deeply plicated specimens (fig. 2c) undoubtedly have a very 



