THORACICA. 31 



tunately, led to give, in my former volume, a description of 

 this kind of the Lepadidae \ and as it is necessary to give a 

 similar account of the other great family of the Order, 

 namely, the Balanidae, I have found it more convenient 

 to make this latter account comparative and supplemental 

 to the former one on the Lepadidae, and so serve for 

 the Order, rather than attempt to give a separate de- 

 scription in full of it. For this latter plan would have in- 

 volved much useless repetition, as, on account of the many 

 exceptions and limitations necessary to almost every state- 

 ment, there is little choice between a description of great 

 length and a mere diagnostic character of the Order, such 

 as I have given above. 



The Thoracica may be divided into three very natural 

 Families, of nearly equal value ; firstly, the Balanidae, or 

 sessile Cirripedes, which may be subdivided into two sub- 

 families, also very natural, the Balaninae and Chthamalinae ; 

 secondly, the Verrucidae, containing only one genus ; and 

 thirdly, the Lepadidae, or pedunculated Cirripedes. These 

 three families differ from each other, besides in mere external 

 appearance, almost exclusively in the relation of the diffe- 

 rent portions of their external covering or carapace, and of 

 the muscles moving such portions. In the Balanidae, the 

 four opercular valves surrounding the orifice leading into 

 the sack, are capable of other movements, besides being 

 opened and shut ; whereas all the other valves are immove- 

 ably united together. In the Lepadidae, the valves answer- 

 ing to the opercular valves, are furnished with a muscle 

 only for shutting them ; whereas the peduncle answering to 

 the basis in the Balanidae is capable of various movements. 

 In the Verrucidae the shell is singularly asymmetrical; only 

 half of the operculum (either the right or the left side, this 

 varying even in the same species) being moveable; the 

 other half being immoveably united to the remaining valves ; 

 and the whole shell has only one muscle serving to shut the 

 moveable half of the operculum. All the internal parts 

 and organs are very similar in the above three Families. 

 If, however, the internal structure of one of the two sub- 

 families, into which the Balanidae may be divided, namely, 

 of the Balanina?, be compared with that of the Lepadidae, 



