308 POLLICIPES POLYMEliUS. 



are not of equal sizes. Viewed externally, the valves 

 seem to touch and overlap each other ; viewed internally 

 (PL VII, fig. 2a) they are found to be just separated from 

 each other by transparent membrane ; none of the valves 

 are articulated together. The outer surfaces of nearly 

 all the valves, except in the two last formed whorls, are 

 much disintegrated, and seem to be composed of alternate 

 white and brown layers of shell. The membrane con- 

 necting the valves, as well as that of the peduncle, (in 

 specimens long kept in spirits,) is brown ; but in some 

 dried specimens, there are indications of its having been 

 coloured crimson (as in P. cornucopia), round the orifice 

 and between the valves. 



Scuta, irregularly oval, convex, narrow at the upper 

 end ; basal margin may be almost said to be formed of 

 three short, unequal margins, corresponding with the 

 rostrum, the rostral and the adjoining latus. The edge 

 corresponding with the latter, is the best marked, and is 

 generally slightly hollowed out, as if a piece had been 

 broken off'. The tergo-lateral margin is curved and pro- 

 tuberant. The umbo projects a little over the scutal 

 margin of the terga. 



Terga, projecting beyond the other valves to an un- 

 usually small degree, broadly oval ; basal angle bluntly 

 pointed, apex rounded, blunt ; scutal margin, hollowed 

 out to receive the upper part of the tergal margin of the 

 scuta ; carinal margin curved and protuberant ; occludent 

 margin consists of two short sides at right angles to each 

 other. The whole valve in length and area is about equal 

 to the scuta ; internally, somewhat concave. 



Carina, triangular, rather narrow, internally deeply 

 concave, very slightly curled inwards ; basal margin pro- 

 tuberant, with a large central portion considerably 

 hollowed out. 



Rostrum, triangular, of nearly the same shape as the 

 carina, but only one third of its length, internally very 

 slightly concave, and with the basal margin various, being 

 either truncated or angularly prominent in the middle. 



