GENUS BALANUS. 191 



admit, that if B. stultus and Ajaoc had never existed, B. 

 calceolus and its three allies might have formed as natural 

 a little group, though difficult to be characterised, as does 

 the sub-genus Acasta; or perhaps this group and Acasta 

 might have been combined together. These same species, 

 viz., B. calceolus and its allies, are intimately allied to B> 

 terebratus and inclusus, which are contained in the last 

 section (f) of the genus ; and this shows that Dr. Gray's 

 proposed genus Chirona, including the species with non- 

 porose parietes in sections (e) and (f), could hardly have 

 been instituted, even if the porosity of the parietes had not 

 been variable in B. unguiformis, balanoides, and glandula. 

 My fourth section (d), founded on the basis not being 

 porose, is perhaps the weakest of the divisions, as may be 

 seen in the list of exceptions appended to the sectional 

 headings. 



The arrangement of the species is, I think, as natural as 

 a linear one could be made : every one knows how irregu- 

 larly and in how many directions the lines of affinity in 

 every natural genus branch out. Some few species stand 

 rather isolated, as B. declivis ; and B. allium, cepa, and 

 quadrivittatus in a little group by themselves. I have 

 already shown how the species in the first and second sec- 

 tions (a and b) blend into each other; and that the 

 blending species are likewise allied to some in the last 

 section (f) ; furthermore, I shall have occasion to show that 

 these same species can hardly be separated naturally from 

 the sub-genus Acasta. The first section, moreover, passes into 

 the third (c) by B. tulipiformis ; and the third into the 

 fourth (d) by B. imptrovisus, nubilus, corrugatus, and patel- 

 laris : the fifth and sixth (e and f) sections are closely con- 

 nected together by B. cariosus and Jlosculus ; and these two 

 sections blend into the fourth (d) through B. unguiformis 

 and balanoides, and lastly, into the third (c) section by B. 

 dolosus and imjjrovisus. 



The genus, as we have just seen, is hardly separated 

 from the sub-genus Acasta ; by B. allium it tends to pass 

 through the sub-genus Creusia into Pyrgoma; and by 

 B. imperator and jloscidus it graduates into Elminius and 

 Tetrad ita. 



