159 



For subdividing this section other reasous may be found later, I feel unable with the material 

 at hand, to make a proposal in that direction. 



Section Striato-Balanus. This section contains species that have no pores in the 

 the parietes and radii (which belonged to Darwin's Section F or might have been considered 

 to belong to it). They show the common features of having the scutum striated loncritu- 

 dinally, and of having, as a rule, narrow radii. These species — B. amaryllis Darwin, B. aldus 

 n. sp., B. öimac n. sp., B. /naat lat 'us n. sp. — seem to form a very natural group. Several 

 species belonging to it are of somevvhat larger size. Whether B. Hameri Ascanius is to be 

 placed in this section or into the next is difficult to say : it has the scutum feebly striated 

 longitudinally, but its radii are not so narrow as is the case in the more typical species. 

 Balanus vestitus Darwin, B. imperator Darwin, and B. jlosculus Darwin have narrow radii also 

 (they are even absent in B. flosculus) and might also be included in this section ; they do 

 not show, however, the characteristic feature of having the scutum striated. 



Section Solido-Balauus. This section contains species which have the parietes, radii and 

 scutum smooth, as a rule rather truck, the parietes and radii, moreover, without pores. They 

 have also a solid basis, with crests running over the surface, or with narrow canals radiating 

 from the circumference. The radii are rather broad and most of the species seem to be of small 

 dimensions only. B. socialis Hoek, ancl B. maldivensis Borradaile belong to this section, which, 

 embraces B. auricoma n. sp., B. ciliatus n. sp., and B. compressus n. sp., of the material 

 collected by the "Siboga". 



Section Membrano-Balanus corresponds to Darwin's Section E and contains the 

 species with a membranous basis. I am in doubt whether this is really a good section, 

 that is, a natural group of species : like Darwin himself I do not feel sure that the species 

 really belong together. That the basis is membranous, or is non-calcareous, is a negative 

 character only. But as only one species of the Siboga-material belongs to it, and is, more- 

 over, a somewhat abnormal form, I had better not enter into discussion about this question. 

 The new Siboga-species is B. longirostrum , which is nearly related to B. declivis Darwin ; 

 it has non-porose parietes and lives embedded in sponges. Numerous segments of both 

 rami of the fourth cirrus, are, in this species, armed with transverse groups of triangular 

 teeth, such teeth occurring also, but smaller, on most of the segments of the 2 nd and 

 3 rd cirri. This shows, perhaps, that the species is allied to some of the species of the Section 

 Ar ma f o-Ba la n us. 



Section Armato-Balanus. The species of this section have as a rule the parietes and 

 radii without pores ; some of its species, which were already known and which I propose 

 to include in it, were considered by Darwin to belong to Section F. This section emerges 

 into the genus Acasta 1 by some of the segments of the anterior ramus of the 4 th cirrus 

 being armed with rows of teeth, and it is at the same time nearly related to the section 



i Darwin has pointed out already that some of the species of his Section F (/.'. tacbratus, B. allium, B. quaJiivittatus) can 



hardly be separated natuvally from the genus Acasia. It is certainly very interesting that Darwin's main reason for separating them 



viz. the structure of the 4th cirrus in Acasta — has now been found not to exist: the structure of that cirrus in these species corresponds 

 exactly with that of some species of Acasta. 



31 



