GENUS VERRUCA. 505 



the occludent (a, see fig. 1 b) and basal (b) 3 can be identified 

 without a doubt in the fixed valve : the third and tergal 

 margin remains ; this should have two articular ridges ; of 

 these the upper one, still serving its normal function, can 

 be detected in all the species (' in fig. 1 b), and can be seen 

 pretty plainly (' fig. 5) in V. necca : but of the lower and 

 other articular ridge there is no sign, — excepting indeed the 

 whole parietal portion of the valve, which, from holding an 

 exactly homologous position with the lower articular ridge 

 of the moveable valve, I cannot doubt in this ridge expanded 

 and curiously metamorphosed. Hence, in both fixed scutum 

 and tergum, it is the outermost or lowest of the articular 

 riclges which has been modified and expanded, so as to rest on 

 and be fixed to the surface of attachment. It would appear 

 as if it had resulted from one ridge in each of these valves 

 having been thus used up by expansion (so to express my- 

 self), that the suture between the fixed scutum and tergum 

 is more simple than any other suture in the whole shell ; 

 and it is owing probably to this straightness, and conse- 

 quent tendency to weakness, that the valves do not grow 

 along this line, and so do not become separated from each 

 other during growth, as on the three other lines of suture. 

 As it actually is, owing to this suture never being separated, 

 it is even stronger than the others ; its edges on the inside 

 (fig. 1 c), I may add, are a little inflected or prominent. 



Rostrum and Carina : these valves differ from each other, 

 only in the former (a) being rather the largest, and in being 

 more plainly articulated with the fixed scutum, than is the 

 carina (b) with the fixed tergum. Their umbones stand in 

 their normal places, at the two ends of the orifice leading 

 into the sack, that is, facing the -dorso- ventral longitudinal 

 plane of the animal ; but they are very unequally developed 

 on the two sides, and hence they rise very obliquely from the 

 surface of attachment. Their summits are nearly square, 

 which is caused by the continued growth on both sides of the 

 oblique plates or ridges, by which they are articulated with 

 the adjoining valves. These plates strikingly resemble, as 

 already stated, the radii in certain species of Chthamalus. 

 Without these articulating plates, the outline of the rostrum 

 and carina would have been triangular, with the apex up- 



