VERRUCA NEXA. 523 



not at all depressed like the former species, but the walls 

 are almost perpendicular or even overhang their bases, and 

 the summit of the shell consequently is broad. This form 

 may be in part, but only in part, due to the attachment on 

 the thin branches of the Gorgonia. The umbones of the com- 

 partments are remarkably prominent and sharp. Although 

 the parietes are nearly smooth, yet from being so steep, 

 they are little seen, and owing to the very prominent but 

 rounded ribs by which the compartments and opercular 

 valves are articulated together, the whole shell has a 

 strongly ribbed appearance. The diameter of the largest 

 specimen was *2 of an inch. 



The rostrum (a, fig. 5) is patelliformed, with the umbo of 

 growth sub-central, but rather above the middle point ; 

 hence this valve, differently from the carina, and differently 

 from the rostrum of the other species, grows not only at its 

 basal margin, and on both sides where opposed to the 

 carina and fixed scutum, but also along its upper margin 

 where opposed to the basal edges of the moveable scutum 

 and tergum : owing to the perpendicularity of this valve, 

 the upper part forms a ledge almost parallel to the orifice 

 of the shell. The carina (b) is of unusually small size, being 

 about only half the size of the rostrum, and scarcely exceed- 

 ing in size the fixed tergum. Thefwed scutum (s') is large, 

 larger even than the carina ; it is oblong, and its shape is 

 more simple than in the other species ; this is chiefly 

 owing to the rostrum articulating with the whole of that 

 margin (b) which answers to the basal margin of the move- 

 able valve ; whereas in the other species (fig. 1 b) it curls 

 beyond this margin, and articulates with the very protube- 

 rant, so-called, parietal portion of the valve. Three or four 

 rounded prominent longitudinal ribs, exactly like the homo- 

 logous ribs on the moveable scutum, run from the apex of 

 the fixed scutum to the basal margin, and their extremities 

 form the teeth by which it articulates, as just stated, with 

 the rostrum. Its upper articular ridge (') is more prominent, 

 and placed much lower down in the suture between it 

 and the fixed tergum, than in the foregoing species. The 

 ledge (o) by which the orifice is kept neatly closed, is 

 here more distinct than in V. Stromia: this ledge is neces- 



