THE SESSILE BAKNACLES. 



133 



angle. Only B. fos among our species has a similar tergum. B. 

 halanus has some aflinitj to B. nubllw by the tapering spur and the 

 colored beak of the tergum. 



B. nulj'dis has a certain external likeness to B. aquila^ but the oper- 

 cular valves are very different, and I have never seen a specimen with 

 any trace of epidermis. It is closely related to B. fos, yet here again 

 the valves differ ^videly in the structure of their inner faces. 



The opercular valves vary in the tint of the inner faces from light 

 or ^varm buff to cinnamon buff; never white. The purple color of the 

 beak and median spot of the tergum is not invariable, being absent in 



Fig. 36.— Balanus nubilis, station 4196, Monterey Bat. a, labkum. 6, 32d segment of cikrus vi. 



c, maxilla, d, mandible. 



an example from Santa Cruz. The tergum varies a good deal in the 

 ratio of width to length. Except in the very young stages, the occlu- 

 dent border is worn away in its upper part, but in the lower part it is 

 broadly inflexed in the adult stage. 



The scutum varies chielly in the degree of concrescence of the ad- 

 ductor and articular ridges, as shoAvn in the figures. The occludent 

 side is conspicuously folded in, forming an inclosed cavity near the 

 base. The little ridge which divides the cavity of the lateral depres- 

 sor muscle varies in prominence and position, and at times may be 

 hardly noticeable. Sometimes it is carried up on the side of the ad- 

 ductor ridge, when it may readily be overlooked altogether. The 

 growth-ridges of the exterior are very minutely and closely crossed 

 by fine longitudinal stride, but in old examples, such as shown in plate 

 31, figures 3, da, the whole surface is removed. 



The basal pores arc irregidar (pi. 30, fig. Ih). The basis is 

 copiously porous at the periphery (pi. 30, fig. Ic). 



4729"— Bull. 93— IG 10 



