250 balanidjE. 



not acquired their longitudinal striae, and internally the adductor ridge 

 lies not so close to the articular ridge as it does subsequently ; hence I 

 for some time mistook the var. assimilis of B. improvisus for the young 

 of B. eburneus. But I found in the latter, that the rami of the first 

 pair of cirri, are always, even in the earliest youth, more unequal in 

 length, and that each segment of the posterior cirri bears a greater 

 number of pairs of spines, there being, even in very minute specimens, 

 seven pairs. Moreover, after having examined scores of specimens, 

 I found I could almost always distinguish the two species by the 

 smoothness and curvature of the summits of the radii of B. improvisus ; 

 I entertain no doubt whatever about the distinctness of the two species; 

 indeed, when both are mature, besides the greater size, striated scuta, 

 &c. of B. eburneus, their general aspect is very different. 



22. Balanus improvisus. PL 6, fig. 1 a — 1 c. 



Shell tohite: radii narrow, with their upper margins smooth, 

 slightly arched, very oblique. Tergum with a longitudinal 

 furrow ; spur with the end rounded. 



Var. assimilis, with longitudinal white hxjaline lines. 



Hab. — England, Scotland, Belgium (?), Nova Scotia, United States, West 

 Indies, Rio Plata, Southern Patagonia, Guyaquil, West Colombia ; attached to 

 wood, shells, rocks, ships' bottoms, from low tidal level to twenty fathoms depth. 



General Appearance. — Shell conical, with a rather large diamond - 

 shaped orifice, moderately or but little toothed ; very smooth ; walls 

 never folded longitudinally ; white, with an extremely thin pale-yellow 

 persistent epidermis. The radii are very narrow, with their summits very 

 oblique, rounded, and smooth ; the epidermis is generally more per- 

 sistent on the radii than on other parts, and this is exactly the reverse 

 of what is common with B. eburneus. The specimens from nearly 

 fresh-water in the R. Plata (hereafter to be mentioned), are brownish, and 

 have undergone a remarkable degree of corrosion, the outer lamina of 

 the walls having been entirely removed to near the base ; hence the 

 external aspect of these specimens is wholly different from ordinary in- 

 dividuals. The var. assimilis has also a very different appearance, 

 owing to the dead white of the walls being relieved by narrow ap- 

 proximate longitudinal hyaline lines, corresponding with and caused by 

 the longitudinal parietal septa being externally visible through the outer 

 lamina of the parietes ; the epidermis on the radii is also of a rather 

 brighter yellow. The largest specimens which I have seen are those 

 from the Plata, and those attached to a ship from the West Indies, and 

 thev had a basal diameter of *6 of an inch : from "4 to ■ 5 of an inch is 

 the more usual full average size. 



Scuta, with the lines of growth but little prominent: articular ridge 



