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ancl the distinction of the different species in each section primarily on the shape and structure 

 of the opercular valves. And I think it is a fact also, that a comparative study of the known 

 species, has shown that the structure of the animal's body can with advantage be used in 

 dividing the said genus into such sections and for separating the species. In this connection, 

 I think it is remarkable, that the structure of the anterior ramus of the 4 th pair of cirri was 

 pointed out by Darwin, as an important character in separating Acasta from Balanus, and 

 that later researches have shown that a similar structure occurs also in numerous species of 

 the latter genus. Supposing he had known this peculiarity, Darwin would certainly not have 

 denied its value for subdividing this genus. 



I will now, first of all, discuss a few species of Balanus for which new sections of 

 the genus were proposed, as Darwin's division seemed inadequate to include them. These 

 cases I think after all, do not or not sufficiently prove the necessity for so doing. 



The first instance is found in my own work. While studying the Cirripedia of the 

 Challenger expedition, in 1882, I met with two species with a membranous basis, but which, 

 by the absence of radii and by the structure of the animal's body, showed themselves to be 

 quite different from the species of section E (to which section they in accordance with their 

 membranous basis should belong), and also from any known species of the genus. I therefore 

 thought there was good reason to propose a new section for the reception of these species. 

 The Siboga material contains specimens of two new forms, which doubtless belong to the 

 same group, and in the mean-time Pilsbry (191 i) published descriptions of two other new 

 species, which appear also to be closely related to the somewhat abnormal forms I described 

 in the Challenger Cirripedia-report. On carefully going over the new material which came to 

 hand however, I found, that there was sufficiënt reason to separate this whole group of species 

 from the genus Balanus and to create for it a new genus. Therefore, my section G of Balanus 

 is to be cancelled ; for further particulars regarding the new genus I refer to what is to be 

 found in the present report, when this new genus (Hcxelasma) is described. 



Borradaile (1. c), in 1903, described a new species of Balanus (B. maldivensis), col- 

 lected in S. Nilandu Atoll (Maldive Archipelago), and found its shell so peculiar, that it appeared 

 to necessitate the founding for it of a new section of the genus. This had the following cha- 

 racters : "All parts of the shell present, heavy and without pores". The Siboga material, which 

 contains this species also, gave me a good opportunity to study the form, and, as I will 

 explain in further detail under the specific heading, made me doubt whether there was really 

 sufficiënt reason for maintaining for it a new section. I think that its place is in Darwin's 

 section F, but there is on the other hand good reasons for subdividing that section. 



In the same year (1903) Gruvel (1. c.) proposed to establish a new section for a spe- 

 cies from the Congo which he named B. Dybowskyi. He defined that new section as follows: 

 "Parietes and basis with pores, no radii ". I have never seen specimens of this species, so I 

 only know it from the figures Gruvel published in his "Monographie" of 1905. The chief 

 character of the section is the absence of radii in Gruvel's new species. Providing these are 

 totally absent, then, I think we may ask if this species does really belong to the genus 



Balanus} We should not forget, however, that there are species in Darwin's section F, 



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