THE SESSILE BARNACLES. 



85 



The original description by Darwin is as follows : 



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 persistent on the radii than on other parts, and this is exactly the 

 reverse of whit is common with B. eburncus. The specimens from nearly fresh 

 water in the River Plata 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 

 individuals. The var. assimilis has also a very different appearance, owing to the 

 dead white of the walls being relieved by narrow approximate longitudinal hyaline 

 lines, corresponding with and caused by the longitudinal parietal septa being exter- 

 nally visible through the outer lamina of the parietes; the epidermis in the radii is 



FjG. 10. a, BALANT7S IMPEOVISUS X 3, QUINNEPIAC, MASS.; 6, B. EBUBNEU3 X 2, OCEAN CITY, MD., TO 



ILLUSTRATE DIFFERENCE IN THE KADII. 



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 they had a 

 basal diameter of O.G of an inch from 0.4 to 0.5 of an inch is the more visual full aver- 

 age size. 



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

 but little reflexed; adductor ridge straight and very prominent, varying a little 

 in its distance from the articular ridge; there is scarcely any depression for the lateral 

 depressor muscle; the upper internal surface of the valve is roughened with ridges. 



Terga, with a moderately deep longitudinal farrow; spur short, rather narrow, 

 with the end rounded, placed at less than its own width from the basiscutal angle; 

 in the Rio Plata specimens the spur is close to this angle; the basal margin is gener- 

 ally straight on opposite sides of the spar, but sometimes on the carinal side is a 

 little hollowed out. The lines of growth are upturned along the carinal margin, which 

 consequently is a little protuberant, but to a varying degree. The crests for the 

 depressors are extremely distinct and prominent. In the varieties having the basi- 

 cariual margin hollowed out, and the carinal margin protuberant, there is a marked 

 resemblance to the peculiar tergum of -B. eburneus. 



Walls: The parietal pores are tolerably large and are crossed by numerous trans- 

 verse septa; the longitudinal septa are very finely denticulated at their bases, but 

 occasionally almost smooth. The radii are, as stated, extremely narrow, and very 

 remarkable from their smooth rounded edges; their septa are barely denticulated. 

 The ake are remarkably protuberant; they have their summits much less oblique 

 than those of the radii, and sometimes they are almost parallel to the basis; their 



4729 Bull. 93167 



