154 BULLETIN 93^ UNITED STATES NATIONAL MUSEUM. 



the wall are more separated and the septa are irregularly spaced, the 

 internal ribs are more numerous, and the lamellae within the outer 

 lamina are stronger. Eostrum has 13 tubes. There is an extraor- 

 dinary development of the interseptal lamellae within the outer 

 lamina of the wall in a specimen from Portland, Maine (pi. 35, 

 fig. 6). In one example from Cork, Ireland, the interseptal lamella? 

 within the outer lamina of the wall are similarly lengthened (pi. 

 35, fig. 7, Cat. No. 12092). These lamella? are minutely crenulate; 

 many of them almost reach the inner lamina. 



Plate 35, figures 2, 2(2, Albatross Station 2070, Georges Bank, in 

 75 fathoms. Wall thin, the tubes compressed. Inner lamina of wall 

 with one to four ribs terminating in strong, denticulate teeth. Outer 

 lamina bordered with crowded, short lamellae. The rostrum has 13 

 tubes. 



Plate 35, figure la, Aberdare Channel, Franz Josef Land, above 

 north latitude 80°. Very few interseptal ribs or lamellae; 14 tubes in 

 the rostrum, which is very narrow. It appears that the number of 

 tubes (about 14 in the rostrum) is rather constant, but their shape 

 and the development of interseptal ribs and lamellae depend upon the 

 thickness of the walls and the shape of the compartment, whether 

 spreading, as in conical individuals, or narrow, as in cjdindric forms 

 of the species. 



There are never any transverse septa in the tubes. 



Individuals growing on Pecten are often beautifully fluted, in 

 harmony with the ribs of the shell (pi. 33, fig. 2«) , while others in the 

 same group may be unaffected. Often the Pecten specimens have only 

 traces of ribs, the surface being minutely shagreened, as though 

 the conflict between the ribs of the barnacle and those induced by 

 the Pecten had resulted in a general flattening of sculpture (pi. 35, 

 fig. 2). The scutum is thick, often strongly curved; pit for the 

 dejoressor muscle narrow, deep, almost tubular from the prominence 

 of the tergal border and the adductor callous. In the single British 

 specimen of the typical form I was able to dissect, the six teeth of the 

 labrum (fig. 43«) are exceedingly small. The sixth cirrus has four 

 pairs of spines on the segments (fig. 43&) instead of five, as stated 

 by Darwin, but it is likely that more distad segments would show 

 another pair. In the dry example examined, the cirri were broken 

 at the eighteenth segment. The mandibles and maxillae are substan- 

 tially alike in the numerous forms of B. halamus and B. I'ostrntns 

 examined, merely differing a little in the proportions and number 

 of the spines of the maxillae. 



Regional distribution and variation. — European specimens (pi. 33, 

 figs. 1-C^) rarely attain 30 mm. in diameter, and are more frequently 

 20 to 25 mm., judging by a considerable series in the museum, brought 

 together by the conchologist J. Gwyn Jeffrej^s. 



