114 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



The size and the sculpture of the scutum are variable among indi- 

 viduals, and especially in different colonies, even in the same general 

 region. Thus, a lot of small trigonus from Misaki, Sagami, Japan, 

 growing on Fusus, has but one row of pits in the scutum, though they 

 were apparently adult. Another Japanese lot from Kagoshima, at 

 the southern extreme of Kyushu, has long, narrow scuta, with several 

 rows of pits. Detailed faunistic studies are needed to determine the 

 significance of such variations. 



It is quite possible that the Atlantic form of B. trigonus (pi. 26, figs.l- 

 12a) can be separated from the Pacific form (pi. 26, figs. 13-13e) as a 

 race or subspecies. The typical Pacific form of trigonus, as figured 

 by Kriiger (Japan) and shown in figure 28d, Cape St. Lucas, has the 

 anterior margin of the segments of cirrus iii more protuberant and more 

 coarsely toothed than in Atlantic examples examined (fig. 28a). The 

 tergum is somewhat wider. If these differences prove constant, when 

 a large series is examined, the Atlantic race may be called B. trigonus 

 armatus. As most of the specimens at my command are dry, I have 

 been unable to satisfy myself that the differences are sufficiently con- 

 stant to have racial value. 



Balanus armatus Fritz Muller, from Santa Catharina Island, 

 Brazil, has been accepted by subsequent authors as a valid species. 

 It is merely a variety of B. trigonus, due to station. It lives on 

 sponges, PapiUina or Reniera, on the polyp Carijoa, or rarely on 

 Purpura, or on rocks, in that case assuming the ordinary ribbed 

 exterior of B. trigonus. According to Muller, who has given a full 

 and beautifully illustrated description, the orifice is pentagonal. 

 The projecting apices of the compartments make the peritreme toothed ; 

 surface generally smooth, more rarely having indistinct ribs. Those 

 seated on rocks have stronger ribs. There are sometimes traces of 

 epidermis on the lower parts of the parietes. Muller states of the 

 terga, " Diese stimmen ganz mit der von Darwin f tir Balanus trigonus 

 gegebenen Beschreibung uberein," but he figures the spur with a 

 more rounded distal end, quite possibly an oversight in drawing. 

 The narrowness of the tergum, as figured, agrees with Antillean 

 specimens. On the west coast the tergum is broader. The scuta 

 are exactly as in a common Pacific form of trigonus. The mouth 

 parts and cirri, as figured by Muller, agree well with those organs 

 in West Indian trigonus, even in the small details, which vary more 

 or less in specimens from differeni localities. Part of Muller' s figures 

 of armatus are copied photographically in plate 26, figures 1 to 11. 



