GENUS VERRUCA. 503 



of the walls of the shell, and may be called the parietal 

 portion. The rostrum (a) curls round the end of the occlu- 

 der^ portion, under an edge (b), evidently answering to the 

 basal margin (5) of the moveable scutum, and is simply 

 united to this portion by membrane, but beyond this part, 

 it is articulated to the parietal portion (") of the fixed scutum, 

 by oblique interlocking ridges, like those forming the suture 

 between the rostrum and carina. In V. nexa, however, 

 (fig. 5) the rostrum (a) does not curl round any part of the 

 interlocking fixed scutum (s'), but articulates with it by a 

 straight suture. Internally, the fixed scutum (s' in figs. le,lc, 

 of reversed shells) has a surprisingly large, thin adductor plate 

 (m in fig. 1 c, 1 b), with a rounded outline, projecting nearly 

 parallel to the basis or surface of attachment ; the adductor 

 scutorum muscle is attached to its upper surface, and conse- 

 quently the animal's body lies between this plate and the 

 moveable scutum. In the fixed scutum of V. nexa, however, 

 there is a deep pit, instead of a plate, for this muscle. 



The fixed Tergum, likewise, consists of two portions — a 

 middle and lower, or parietal portion, and a rim or upper 

 portion ; the rim consists of two unequal arms, answering 

 to the two upper margins (not merely the edges) of the rhom- 

 boidal moveable tergum ; the longer rim [x in t', see fig. 5) 

 answers to the carinal margin (x) of the moveable tergum, 

 and may be called the carinal rim ; and the shorter rim 

 (o and ' in fig. 5) answers to the occludent margin {p and ') of 

 the moveable valve, and may be called the occludent rim. 

 The carina curls round the end of the carinal rim, under an 

 edge, z (much fore-shortened in t' in fig. 1 b, and best 

 seen in fig. 5), answering to at least a large part of the basal 

 margin (z) of the moveable tergum, and interlocks, by a ser- 

 rated suture, with the edge of the parietal portion of the 

 valve. Internally (fig. 1 e, less plain in 1 c) there is a trans- 

 verse ledge, notched in the middle, and sometimes deeply 

 hollow beneath, running across the valve in about the line 

 of the adductor plate of the fixed scutum : this ledge, in fact, 

 marks and is partly caused by, the line of separation between 

 the central or parietal, much modified, and the scarcely 

 modified, upper or rim portion of the valve. The use of this 

 ledge is apparently to give attachment, as does the under 



