502 verrucidjE. 



scutum, there are only two ridges, though, as before noticed, 

 an outer indentation, which is developed as a ridge in V. 

 nexa (fig. 5, s), might almost be counted as a third articular 

 ridge. 



I may here just remark, that the furrow between the two 

 ridges on the tergal margin of the scutum, resembles the 

 articular furrow in the scutum of the Balanidse ; but it may 

 be doubted whether the resemblance be more than super- 

 ficial, as this furrow, in the case of Balanidae, receives the 

 edge itself of the tergum, whereas here it receives only a 

 ridge, proceeding from the apex of the tergum, to a nearly 

 middle point on its scutal margin. Finally, 1 may add, 

 that the tergum in this genus, in general shape, in growth 

 (presently to be referred to), in the manner in which the 

 upper scutal shoulder is overlapped by the scutum, and in 

 the presence of the axial ridge, presents a very striking re- 

 semblance to certain old fossil species of Pollicipes, and to 

 a limited extent to the living species of Lithotrya. 



The scutum and tergum being interlocked, move to- 

 gether ; they can be firmly shut by the contraction of the 

 long adductor scutorum muscle. Their opening appears 

 partly due to the elasticity of the membranous hinge (re- 

 presenting the opercular membrane), by which they are 

 attached transversely, just beneath the summit of the carina 

 and rostrum. No doubt the protrusion of the cirri effec- 

 tively aids the act of opening. These valves are not capable 

 (nor, of course, the other valves) of any other movement ; 

 for there are no muscles for such movements. 



Fixed Scutum and Tergum. — The fixed scutum is larger 

 than the fixed tergum, and therefore has the same propor- 

 tions as the homologous valves in ordinary cirripedia, but 

 reversed proportions compared with the moveable scutum 

 and tergum. The shape of neither valve can hardly be 

 described. The fixed Scutum (s'in all the figs.), externally, 

 seems at first to consist of two portions, namely, a curved 

 occludent rim (a), closely resembling the opposed occludent 

 margin (a) of the moveable scutum (s), having in fact under- 

 gone very little modification, (as may be best seen in PI. 21, 

 fig. 1 b) ; and secondly, of a much modified portion (marked 

 "), which resembles in outline and state of surface the rest 



