TETRACLITA POROSA. 331 



Scuta : these are sub-triangular and generally a little elongated, but 

 they vary slightly in relative breadth (fig. 1 i, 1 I), and likewise in the 

 degree to which the basi-tergal angle is rounded off. The under surface 

 is clouded with dull or pinkish-purple, or with green, or is nearly white. 

 The articular ridge is not prominent, and the articular furrow is narrow. 

 The adductor ridge is prominent, and runs upwards for some distance 

 close and parallel to the articular ridge ; and sometimes it extends 

 nearly or quite up to the apex of the valve ; in one single specimen 

 the adductor ridge had an abraded appearance, and was very little 

 developed. The crests for the rostral and lateral depressores are sharp 

 and distinct. Along the occludent margin, the ends generally of the 

 alternate lines of growth are enlarged into knobs, serving to lock the 

 two valves together ; but in many specimens only two or three knobs, 

 at intervals of several lines of growth (fig. 1 b), are developed. 



Terga : when the upper end of the valve is not corroded, there is a 

 distinct beak, hollow within for a thread of corium. The scutal 

 margin is not much inflected, and the articular ridge not very pro- 

 minent. The spur is placed quite close to the basi-scutal angle of the 

 valve, so that there is no basal margin on that side of the valve. Te 

 width of the valve and of the spur, and the acumi nation of the ex- 

 tremity of the latter, varies in a remarkable manner. In the broad 

 and commonest variety (fig. 1 k), the basal margin of the valve form 

 an angle of about 130° with the carinal side of the spur, and the basal 

 end of the spur is broad and truncated. In the less common and 

 narrow variety (1 m), the basal margin in some extreme cases forms 

 very nearly a straight line with the carinal side of the spur ; and the 

 spur itself is bluntly pointed : in var. 7 it is sharply pointed. 



Structure of the parietes and radii. — In all cases the four sutures 

 are quite distinct, from top to bottom, on the internal lamina of the 

 shell, and generally they run through the whole thickness of the walls, 

 and are visible externally. Often they do not quite reach the outer 

 lamina, and then the shell externally consists of a single piece, like a 

 patella. Sometimes the sutures can be traced running through the 

 parietal tubes only for a short distance from the internal surface ; where 

 they cease, the two walls of the suture become fused together. When 

 a perfect suture is split open, the radius is represented (fig, I h) by a 

 few narrow, sinuous ridges, sending out on each side little branches or 

 denticuli; these are received into corresponding furrows in the opposed 

 compartment. These ridges run nearly parallel to each other, and some- 

 what obliquely, from the outer lamina of the shell to the basis. When 

 the radii are developed, their edges are similarly formed, by sinuous 

 denticulated ridges, with the interspaces between them filled up solidly. 

 The alee are but little prominent. 



The mouth does not deviate from the generic type. The cirri are 

 remarkable from the variability in the several pairs of the relative num- 

 bers of their segments, as shown in the following table. The segments 

 do not correspond even on opposite sides of the same individual, as may 

 be seen in the two lower lines of the table. I believe that variability 

 to this degree is very uncommon in other cirripedes, though, as stated 

 in the Introduction, the number of the segments always increases with 



