THE SESSILE BARNACLES. 



147 





The most profusely spinose examples of Z>. /. apertus were taken 

 from a "hermit crab sponge," Captains Bay, Unalaska (pi. 36, 

 fig. G). The base is very concave, and the spines remind one of 

 spinose varieties of B. tintinabulum. Opercular valves, pink-tinted. 

 The sponge which these barnacles grow in is a very close-grained, 

 soft and friable species. The typical form of apertus is almost al- 

 ways on sponge, but sometimes on shells. One from Pccten (Cap- 

 tains Bay, Unalaska, 10 fathoms, Dall) reproduces the ribs of the 

 shell and has no spines (pi. 36, fig. 4). Another, growing on 

 TerebratyMna (pi. 36, figs. 3, 5), Albatross Station 2849, is unusually 

 thin, with smooth walls and very large orifice. A ground specimen 

 is figured to show the transverse septa of the upper half of the 

 parietal tubes, and their absence in the lower half. A rostrum of one 

 of this lot has 18 tubes ; there is a single accessory rib in each inter- 

 septal interval. In the specimens growing on shells the base is less 

 concave than when growing on sponges, conforming, of course, to 

 the contour of the shell. 



When growing on a sponge, it appears that the pressure of the 

 growing basal edges of the Avail tends to cut into the sponge, so 

 that the periphery penetrates deeper than the initial point of 

 fixation, causing the base to be dome or cone shaped. In Acasta 

 the exactly opposite condition obtains; the growing periphery of 

 the barnacle yields to the pressure of the living sponge and is forced 

 upward, the base becoming an inverted cone, or bowl-shaped. 



BALANUS ROSTRATUS DALLI, new subspecies. 



Plate 38, figs. 1-lc, 2. 



This form attains a large size and is quite solid; external surface 

 smooth; radii very little sunken, moderately wide. Parietal tubes 

 large and square, with one or two interseptal ribs on the inner lamina; 



