GENUS VERRUCA. 499 



brane, though lying on one side of the animal, is the true 

 basis. To make all the parts in Verruca hold the same posi- 

 tion as in other cirripedes, relatively to the surface of attach- 

 ment, we must develope the carina and rostrum equally on 

 both sides of the true longitudinal axis of the shell, and insert 

 the newly-developed portion between the basis and the fixed 

 scutum and tergum, reducing the latter in size, and tilting 

 a little up the moveable scutum and tergum ; and by this 

 means the animal's body would be turned, so that its dorso- 

 ventral longitudinal plane would stand at right angles to 

 the basal membrane. 



Extraordinarily great as is the difference between the 

 right and left sides of the whole shell, yet in all the species 

 it seems to be entirely a matter of chance whether it be the 

 right scutum and tergum with the right side of the rostrum 

 and carina, or the left scutum and tergum with the left side of 

 the rostrum and carina, which become abnormally developed. 

 Nor does there seem to be any relation between the side 

 of the operculum to be attached, whether right or left, and 

 the nature of the surface of attachment; for I have seen 

 many specimens adhering to perfectly level surfaces, and to 

 quite cylindrical branches of Laminariae ; and in these cases, 

 however the larva might attach itself, there could be nothing 

 to favour the development of one side more than the other. 

 Although the attached scuta and terga are larger than the 

 moveable pair, yet, owing to the small development of the 

 carina and rostrum on the attached side, the upper or un- 

 attached side must be considered as the most developed. In 

 this respect, and in the circumstance of either right or left 

 side being modified, we are reminded of the structure of 

 Pcecilasma Kmnpferi (described iir my former volume on 

 the Lepadidse), in which the valves on the side of the capi- 

 tulum, nearest to the crab's body, to which the specimens 

 were attached, were somewhat less developed than those on 

 the opposite side. I may add, that in ordinary Crustaceans, 

 as I am informed by Professor Bell, the unequal develop- 

 ment of the thoracic limbs seems quite capriciously to affect 

 either the left or right side of the body. 



General Appearance of the Shell. — The shell is in most 

 cases much depressed and irregularly circular; the side formed 



