THE SESSILE BARNACLES. 297 



e 2 . Tergum narrower in the lower half, the basal margin distinctly 



sinuated or incurved helow the depressor muscle crests. 

 p. Crests of the tergum on a little plate raised from the valve; 



sutures iuterfolding when perfect C. scabrosus (p. 323). 



f. Crests of tergum normal ; sutures simple C. challcngeri (p. 307). 



d 3 . Scutum without a distinct adductor ridge. 



e 1 . Greatest prominence of articular ridge of the scutum helow middle 



of tergal margin, its lower end ahruptly truncate. 

 /'. Occludent margin of scutum making a right angle with edge of 

 articular ridge; tergum very broad above, triangular, its basal 



margin not sinuated below the crests C. stellaiiis (p. 301). 



f. Occludent margin of scutum making less than a right angle with 



the tergal margin. 

 g 1 . Tergum triangular, broad above, the basal margin straight. 



C. anlcnnfilits Darwin. 

 g". Basal margin of tergum distinctly sinuated below the crests. 



h l Tergum very narrow and thick C. s. (nigustiterguni (p. 305). 



h 2 . Tergum moderately narrow, having a long furrow next to the 



crests f. s. lisinuatus (p. 306). 



h 3 . Tergum wide above, narrow at the base. 



C. s. punctatus (p. 304). 



7i 4 . Tergum wide at the base C. moro (p. 311). 



e". Greatest prominence of the articular ridge of the scutum about 



median or above, the lower end gradually sloping. 

 f. Sutures zigzag. 



g 1 . Exterior ribbed C. dcntatus (p. 294). 



(f. Exterior nearly smooth G. caudatus (p. 314). 



/ = . Sutures simple. 



(f. Tergum broad and rounded at base ; wall usually not corroded, 



smooth or with rounded ribs C. fragiHs (p. 297). 



g*. Tergum almost equally wide above and below, with an external 

 furrow ; adductor scar of scutum deep C. panamensis (p. 319) . 

 g 3 . Tergum narrow in the lower half. 



/i 1 . Scutum slender, its greatest width about half the length, the 

 tergal and basal margins about equal ; articular ridge very 



small; tergum triangular in section C. withersi (p. 312). 



7i 2 . Tergal margin of scutum decidedly shorter than the basal. 



C. malaycnsis (p. 310) ; C. cirratus (p. 321). 



ATLANTIC AND MEDITERRANEAN SPECIES. 

 CHTHAMALUS FRAGILIS Darwin. 



Plate 70, figs. 1 to 4. 



1854. Chthamalus steUatus var. fragilis DARWIN, Monograph, p. 456, pi. 



IS, fig. Id. 

 1909. CMhamalus stcUat-us var. fragilis Darwin, STJMNER, Science, vol. 30, 



pp. 373-374. 



Type. British Museum, from Charleston, South Carolina, on 

 oysters. 



Distribution. West Indies, and on the mainland north to Woods 

 Hole, Massachusetts. 



