GENUS VERRUCA. 497 



Creusia. Lamarck. Animaux sans Vertebres, 1818. 

 Ochthosia. Ranzani. Memoire di Storia Nat., 1820. 

 Lepas et Balanus Auctorum. 



Distribution, Northern Europe, Mediterranean, Red Sea, Madeira, West 

 Indies, Tierra del Fuego, Chile, Peru. 



The shell in this genus is extremely unsymmetrical, not 

 two of the six pieces of which it is composed quite resem- 

 bling each other. At first it appeared hopelessly difficult 

 to identify, in a homological sense, these six valves, with 

 those of ordinary cirripedes, but the difficulty soon quite 

 vanished. The operculum consists of two moveable valves 

 on one side, namely, a scutum and tergum, but without any 

 moveable valves on the opposed side : the scutum, though 

 remarkable from being much smaller than the tergum, can 

 be easily recognised by giving attachment to the animal's 

 body and to the adductor scutorum muscle. The four other 

 pieces are articulated together, and form the shell surround- 

 ing the sack, in which the animal's body is enclosed : of 

 these, the two against which the moveable scutum and 

 tergum shut, are smaller, differ greatly in shape, and are 

 articulated together in a different manner from the remain- 

 ing two pieces ; from these facts alone there would be a 

 strong presumption that they were of a different nature. 

 The fixed valve, against which the scutum shuts, is either 

 furnished with a remarkably prominent plate (a in fig. 1 c, 

 s'; compare this with s' in the reversed shell in fig.l e), or is 

 hollowed out, as in V. nexa, for the attachment of the 

 adductor scutorum muscle. Thus it is rendered probable 

 that this fixed valve is a modified scutum ; but a surface of 

 attachment for one end of the adductor muscle might, per- 

 haps, have been developed on any other valve, or a scutum 

 might have become fused with a lateral valve of the shell ; 

 the shell on this latter view being rendered in idea more 

 symmetrical. But when a very young specimen is care- 

 fully examined, it is found that the moveable and fixed 

 scutum, the moveable tergum and its opposed valve or fixed 

 tergum, at the first period of calcification, resemble each 

 other quite closely ; but that, as each zone of shell is added, 

 the differences become rapidly greater and greater : hence, 



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