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The fifth and si.xth pairs are nearly equal. They have respectively 25 and 26, and 

 26 and 28 segments in the two rami. The anterior face of most segments is furnished with four 

 pairs of hairs (PI. XX, fig. 16): one pair (the most distal one) is longer, the second is already 

 much shorter, the third is short, the fourth extremely minute. 



Penis lone, longer than the cirri. Surface covered with numerous hairs, on the distal 

 part especially. Hairs standing off transversely in a peculiar way. No sharp point observed 

 dorsally on the basal part of the penis. 



This species was discovered by Dr. G. C. J. Vosmaer in Sponges belonging to the 

 species Spirastrella piirpurea (Lmk.) Rdl., dredged at the following Stations : 



Stat. 86. June 18/19, 1899. Anchorage off Dongola, Palos-bay, Celebes. Depth 36 m. Bottom: 



fine grey mud (river mud). 

 Stat. 313. February 14/16, 1900. Anchorage East of Dangar Besar, Saleh- (or Sapeh-)bay. 



Depth up to t,6 m. Bottom: sand, coral and mud. 



General Rem ark s. This species occurs in numerous specimens in the sponges it 

 inhabits and will also be found, most probably, at other places in the Archipelago. Weltner 

 (Verzeichniss, 1897, p. 270) mentions B. declivis Darwin as collected by Martens at Batjan 

 (Molucca's), but I think it possible and even probable that it was not Darwin's species but 

 the species here described. I think the difference in shape of rostrum and scutum, and, more- 

 over, the narrowness of the carinal-latus, of sufficiënt importance for not considering the species 

 of the West-Indies and that of the East-Indian Archipelago as identical, however nearly related 

 they may be. I think that the Siboga-species differs also from Pilsbry's B. orcutti which was 

 described (Pilsbry, Henry A., Notes on some Pacific Cirripedes. Proceed. Acad. of Sci. Phila- 

 delphia, 1907, p. 361), on specimens from San Ysidro, Lower Californiaj and which Pilsbry 

 considers as diftering from B. declivis Darwin of the West Indies. The shape of the opercular 

 valves, and that of the rostrum of Pilsbry's species, is certainly different from that of the 

 same parts in my species. The three species, however, are no doubt nearly related : they may 

 be found to be local forms of the same species after all. 



Darwin considered his B. declivis as belonging to the section E (species with membranous 

 basis). I pointed out, under the head of the Genus Balanits, that I consider this classification 

 as more or less artificial. If we take into consideration, not one special character of the forms, 

 but their whole structure — the shell as well as the animal itself — we can corae to another 

 arrangement of the species, and one which, perhaps, gives a better idea of their affinities, so 

 far we can judge at present. 



6. Sectio: Armato-Balanus 



16. Balanus terebratus Darwin. PI. XX, fig. 17 — 21. PI. XXI, fig. 1 — 3. 



DARWIN, Cil, Monograph. Balanidae and Verrucidae. 1854, p. 285, pi. 8, fig. 2a — 2b. 



Darwin gave a description of this species and proposed a name for it, without being 

 able to examine the opercular valves ; but, he said, the species here named is so peculiar 

 that it would have been a fault to pass it over. There is but a single specimen in the 



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