THE SESSILE BAENACLES. 



131 



there is no distinct notch below the upper pair of spines, and there is 

 an enlarged spine (as in B. balanus) on the lower prominence of the 

 edge. 



The very narrow, purple-beaked tergum, the whiteness of the very 

 strong walls, and the bifid alse readily separate B. aquila from all 

 varieties of the Protean B. concavus. It is a very distinct species. 



Besides the specimens recorded below, there is one labeled " Ore- 

 gon," without definite locality or record of collector. Its occurrence 

 so far north needs further evidence. 



Series of B. nubilis. 



BALANUS NUBILIS Darwin. 



Plate 30, tigs. 1-t ; plate 31, figs. 3, 3a 4, 5. 



1854. Balanus mibills Dakwin, Monograph, etc., p. 253, pi. 6, figs. 2a^2c. 

 Type. — British Museum, from Monterey Bay, California. 

 Distribution. — Southern boundary of Alaska to Santa Cruz, 

 California. 



Darwin's original description is as follows : 



General appearance. — Shell conical, nigged, sometimes furnished with sharp 

 longitudinal ribs ; dirty white. Orifice not large, oval, toothed. Radii rather 

 narrow, with their summits oblique, much jagged. Basal diameter of largest 

 speciiuen 2.1 ; height only 1.3 of an inch. 



Scuta broad, with the lines of growth prominent; internally, articular ridge 

 very little prominent, sometimes hardly developed, but thick, ending down- 

 wards in a small fx*ee point. Adductor ridge prominent, blunt, produced straight 

 downward, making a deep longitudinal cavity for the lateral depressor muscle ; 

 in some specimens this cavity is almost arched over, so as to tend to be tubular, 

 with a short ridge in the middle ; in other specimens there is no trace of this 

 tubular structure. 



Terga, with the apex beaked, beak triangular, dull purple; the longitudinal 

 furrow is so shallow as hardly to exist. The basal margin slopes down on 

 both sides, with a nearly equable curvature toward the spur ; hence the spur 

 is broad in its upper part and narrow at its obliquely truncated lower end. 

 Internally, there is an elongated dark purple patch. The shallow articular 

 furrow is of quite remarkable breadth. The articular ridge is medial, and the 

 inflected scutal margin is not wide. The internal surface of the spur is 

 formed into a ridge, which runs a little way up the valve, and is sometimes 

 partially separated from the spur itself, making the basal extremity toothed 

 or double. The crests for the depressors are pretty well developed. 



