506 VERRUCIDjE. 



wards. In V. nexa, in which the walls of the shell are 

 almost perpendicular, the rostrum (a, fig. 5) is very peculiar 

 and patelliformed, with the umbo sub-central : this results 

 from the development of a border at the upper end of the 

 valve. In this same species, the basal edges of the rostrum, 

 carina, fixed scutum and tergum, are rectangularly inflected, 

 so as to form a ledge round the basis, as in the case of some 

 few species of Chthamalus, — the ledge appearing like part 

 of the real basis. During the growth of the shell, the upper 

 internal ends of the carina and rostrum are either rendered 

 solid, or a ledge is formed on the inside across their summits, 

 hollow beneath, like the sheath of the Balanidee, to which 

 solid or hollow ledge the basal margins of the moveable 

 scutum and tergum are attached by a rim of membrane, 

 forming a hinge. 



Direction of Growth : Minute Structure of Valves. — The 

 shell grows downwards all round its basal margin. As 

 far as the diametric growth of its upper part is con- 

 cerned, there may be said to be only three valves, for the 

 fixed scutum and tergum never become, as already stated, 

 separated ; on the three other lines of suture, the valves 

 are added to on both sides ; and thus the whole upper 

 part of the shell, and the orifice, increases in diameter. 

 The moveable scutum and tergum grow along their basal 

 margins, and along the margins by which they are arti- 

 culated together; but the scutum in this latter respect, 

 less than the tergum. The summits of the moveable scutum 

 and tergum, during continued growth, become either worn 

 away, or they project freely; in this latter case, an internal 

 ledge is added round the upper end of the fixed scutum and 

 tergum, so as to keep the orifice accurately closed. In 

 V. nexa the rostrum, with its sub-central umbo, is anoma- 

 lous, as already stated, owing to a broad upper internal 

 border growing in a direction almost directly opposed to the 

 basal growth of the moveable opercular valves. 



In young specimens, on the apices of both scuta and both 

 terga, and on the carina, but not on the rostrum, primordial 

 valves may be distinguished, resembling the valves, so 

 called, which first appear (p. 129, Introduction) after the 

 metamorphosis in, the Lepadiclae. In the Verrueidae, how- 



