1 1 2 PHOLADIDiE. 



about two-fifths the distance from that extremity, without 

 any intervening- radiating channel, suddenly diminishes in 

 convexity. The anterior space is more or less closely set 

 with radiating lines and concentric elevated striae, of which 

 the former are generally the more strongly marked, and the 

 latter in typical examples are usually remarkably delicate. 

 Neither of these traverse the entire surface ; but, after 

 muricating by their intersection the gibbous portion of the 

 shell, become obsolete shortly after passing it, and leave the 

 posterior third smooth, or merely wrinkled with the lines of 

 increase. The sides, although unequal, are less so than is 

 usual with the Pholades ; the posterior is attenuated, but 

 rounded at its extremity (and more rarely abbreviated, 

 scarcely tapering, and broadly rounded at its termination,) 

 its arcuated dorsal and convex ventral edges sloping with 

 almost equal inclination. The anterior end forms a short 

 and obtuse beak, the ventral edge, which encloses an hiatus 

 which occupies one-half the length of the shell, being 

 greatly incurved, and the dorsal moderately but decidedly 

 sloping, and very slightly retuse. The accessory valve is 

 solitary, umbonal, excessively fragile, and resembling that of 

 P. Candida., but is proportionally narrower, and the central 

 excavation almost or entirely invisible. The hinge-margin, 

 which is reflected slightly over the umbones and anterior- 

 ward, but not generally appressed, is internally provided 

 with a callous tubercle ; but is destitute of teeth. The 

 subumbonal blade is slender, curved, and not concave, and 

 expands but little at its apex. 



Animal elongated, white; body oblong; mantle white 

 anteally, tinged with fawn-colour posteally, open centrally 

 at the widest part, for the passage of a thick, nearly round, 

 or widely oval truncated white foot. Line of suture of sides 

 of mantle anteriorly strongly marked by a whitish band. 



