176 BALANID/E. 



imperfect; each branchia composed of two plicated folds ; 

 shell attached to living vertebrata. 



Genera. — Coronula ; Platylepas j Tubicinella ; Xeno 

 balanus.* 



The Balanidae may be divided into two sub-families ; 

 namely, the Balaninae and Clithamalinae ; and, in the former, 

 the genera, as we see, may be very naturally grouped into 

 two sections. The Balaninae differ from the Clithamalinae, 

 as far as the shell is concerned, in the rostrum having radii 

 but no alge, all the lateral compartments having both radii 

 and alae ; on the other hand, in the Clithamalinae, the 

 rostrum has alae, and the rostro-lateral compartments 

 radii on both sides, and therefore no alae. These 

 differences probably arise, as already explained, from the 

 perfect confluence, in the Balaninae, of the true rostrum with 

 the rostro-lateral compartments. In Chelonobia, belong- 

 ing to the Balaninae, we see an intermediate state, with the 

 fusion not quite effected: on the other hand, in one genus 

 amongst the Clithamalinae, namely, Pachylasma, w T e must 

 look to the shell at a very early age, to find the rostrum 

 with its alae, distinct from the rostro-lateral compartments. 

 In Tetraclita, Elminius, and Creusia, the carino-lateral com- 

 partments are aborted, or possibly confluent with the lateral 

 compartments, making altogether only four : in Pyrgoma 

 all the compartments are fused together and form a solid 

 ring. The sub-genus Acasta is, in one sense, very natural, 

 as it includes species most closely allied : in another sense 

 it is far from natural, as some of the species can hardly be 

 distinguished from those species of Balanus, which live 

 attached to Gorgon iae : I almost regret I did not merge 

 the species of Acasta into Balanus. In the Balaninae 

 generally the parietes are either porose, or are furnished on 

 their internal surfaces with regular ribs, representing the 

 longitudinal parietal septa, which in other species form the 

 tubes or pores; there are, however, many exceptions to this 

 rule in several species of Balanus, in Acasta and Elminius, 



* At the end of the volume a Synopsis is given, which will serve as a 

 systematic index for the discovery of generic and specific names. 



