THE SESSILE BAENACLES. 297 



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

 sinuated or incurved below the depressor muscle crests. 

 f. Crests of the tergum on a little plate raised from the valye ; 



sutures iuterfoldiug when perfect C. scabrosiis (p. 3U3). 



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



d^ Scutum without a distinct adductor ridge. 



e\ Greatest prominence of articular ridge of the scutum below middle 

 of tergal margin, its lower end abruptly truncate. 

 f. 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. stcUatns (p. 301). 



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

 the tergal margin. 

 g^. Tergum triangular, In-oad above, the basal margin straight. 



C. (intc)inati(s Darwin. 

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



h^ Tergum very narrovv' and thick C. s. cm gust iter gum (p. 805). 



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



crests C. s. hisini(atvs (p. 306). 



7i^ Tergum wide above, narrow at the base. 



C. s. punctatiis (p. 304). 



]i\ 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^. Exterior ribbed C. dcntatus (p. 294). 



■ g'. Exterior nearly smooth G. caudatus (p. 314). 



/". Sutures simple. 



g^. Tergum In-oad and rounded at base ; wall usually not corroded, 



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



g". Tergmn almost equally wide above and below, with an external 



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



g^. Tergum narrow in the lower half. 



/i\ 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. ivithersi (p. 312). 



/r. Tergal margin of scutum decidedly shorter than the basal. 



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



ATLANTIC AND MEDITERRANEAN SPECIES. 

 CHTHAMALUS FRAGILIS Darwin. 



Plate 70, figs. 1 to 4. 



1S54. ClithamatuH sicllattis var. fragUis Darwin, IMonograph, p. 45G, pi. 



IS, fig. Id. 

 1909. Clitliamalus stcllatiis var. fragitls Darwhi, Sumnkk, Science, vol. 30, 



pp. 373-374. 



Tyipe. — British Museum, from Charleston, South Carolina, on 

 oysters. 



Dlstribvtion. — West Indies, and on the mainland north to Woods 

 Hole, Massachusetts. 



