190 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



The scutum has low growth ridges, the lower ones fringed with 

 the remains of former opercular membranes, and usually a very 

 minute, weak, longitudinal striation. Inside there is a rather small, 

 well-reflexed articular ridge, which is continued downward in a 

 sharp, high, curved adductor ridge in some specimens ; in others the 

 adductor ridge is very weak. The pit for the depressor muscles is 

 deep, and rather large, and often is divided by one or two incon- 

 spicuous ridges. There are several very oblique, coarse teeth on the 

 occludent margin. 



Tergum is very narrow, beaked, the beak often purplish. Furrow 

 to the spur narrow or closed. The articular ridge is long and acute. 

 /Spur very narrow and long, acute, continued upward as a raised 



FIG. 59. BALANUS CARIOSUS, UNALASKA. a, MAXILLA. !>, MANDIBLE, c, LABRDM AND 

 PALPUS. <7, BALANUS BALANOIDES, MIDDLE SEGMENT OF GTH CIRRUS (SEE p. 186). 







ridge on the inside of the valve. Crests for the depressor muscles 

 are strongly developed. 



The basis is wholly membranous. 



The size is rather variable. Unalaska specimens measure : 



Diameter, 56 mm.; height, 32 mm. (largest conic example). 



Diameter, 57 mm. ; height, 37 mm. 



Diameter, 30 mm. ; height, 24 mm. (conic). 



Diameter, 35-40 mm.; height, 50 mm. (cylindric). 



Diameter, 35 mm.; height, 100 mm. (cylindric, longest seen). 



The first measurement is very close to that of Pallas's figures of 

 the type. The third measurement is a more usual size. 



The labrum has two teeth on each side of the median notch ; outer 

 angles are more abrupt than usual. Palpi nearly covered with short 



