THE SESSILE BARNACLES. 151 



Mouth. — Labrum with six teeth ; mandibles witli the fourth and fifth teeth 

 small and rudimentary. Jiaxilhis, with a small notch under the upper pair of 

 spines ; in the lower part there is a single large spine. Cirri, dark brownish 

 purple, making a singular contrast with the white operculum and shell ; first 

 pair, with one ramus having 26 segments, and about twice as long as the 

 shorter ramus having 12 or 13 segments, with their front surfaces protuberant. 

 In the second pair of cirri the segments are but little protuberant ; third pair 

 about one-third longer than the second pair ; sixth pair elongated, having in 

 the same individual 46 segments; these segments have shield-shaped fronts 

 bearing 5 pairs of spines, with some minute intermediate bristles. There is the 

 usual point at the dorsal base of the penis.^ ( See p. 152, fig. 43. ) 



The third (fig. 43<?) and fourth (fig. 43(?) pairs of cirri have 

 minute spinules on the outer faces of part of the segments. 



Nomenclatm^e. — Darwin accepted da Costa's name Balanus yorca- 

 tU'S for this species in order to avoid tautonymy, the earlier specific 

 name Lepas halcvnus becoming Balanus halaims in the new combi- 

 nation. This custom, sanctioned by the general usage of Da ravin's 

 time, has long ago been discarded. The specific name is now held 

 to be the permanent part of the binomial combination. Most of 

 the early authors correctly identified the species Linnaeus described as 

 Lepas halanus. His original specimen is preserved in the collection 

 of the Linnean Society of London, and is figured on plate 33. figures 

 1, la. 



Afjinlties and differential characters. — B. halanus is very distinct 

 from all other Atlantic species. The long-beaked terga. with a wide 

 but rather long spur, the fine longitudinal striation of the scuta, the 

 square tubes of the wall, which has more numerous ribs on the internal 

 surface thanthere are septa in the hctscd edge., and, finally, the solid, 

 never porous, calcareous basis, are amply diagnostic characters. Any 

 single wall compartment or opercular valve will serve to identify it. 

 Usually it may be known also by the wide, level-topped radii and 

 strongly ribbed walls. Some specimens of B. crenatus resemble B. 

 halanus superficially. In the Pacific it is different. There we have a 

 very closely related species in B. rostratus., which occurs with B. 

 halcvnus in Bering Sea; but the two differ in several important de- 

 tails of structure, as follows: 



B. halanus. B. rostratus. 



Parietal tubes without transver.se Parietal tubes having numerous trans- 

 septa, verse septa. 



Spur of tergum wide at the end, its Spur narrow distally, its fasciole nar- 



fasciole broad. rower. 



Adductor ridge of the scutum wanting Adductor ridge distinct, and, though 



or weakly developed. often rather weak, it stands free of 



the articular ridge. 



Penis short. Penis long. 



1 It is not known wbcther Darwin's description of the moutli and cirri were from the 

 strong or the rather weak ribbed form of the species, both of which occur in the British 

 Islands. 



