286 OSTREADJE. 



much more frequent use of the dredge, have rendered 

 this bivalve a comparatively common one, and furnished 

 us with connecting links between the two chief varieties. 



The shape is suborbicular, but rather broader than 

 long, the texture moderately strong, and the surface dull, 

 or scarcely at all shining. Both valves are convex (yet 

 not quite equally so), and typically rather compressed ; 

 they are alike in sculpture, and frequently in colouring 

 also. The exterior appears, under the lens, to be co- 

 vered with most minute subradiatingly divergent curved 

 wavy striula?, which again, under a still higher power, 

 exhibit microscopical concentric lines (especially in the 

 more convex valve), decussating them either wholly or at 

 the sides. Sometimes the surface is otherwise smooth, 

 sometimes there is a marginal belt of narrow and very 

 closely disposed depressed radiating costellae ; occasionally 

 these latter extend over the whole shell with or without 

 the presence of about five principal radiating ribs, which 

 are rounded, generally broad, and variable in amount of 

 elevation. At times the costse are alone present, the 

 costellae, or raised sulci, being obsolete ; more frequently 

 two or three of these intervene between each rib. The 

 auricles are excessively unequal, well defined, scarcely 

 subrectangular, costellated, unarmed, and sometimes lat- 

 ticed ; the anterior one is large ; the posterior one, whose 

 upper edge is not half the length of the front one, is 

 almost rudimentary. The auricular sinus is very narrow, 

 forming a very acute angle at its summit ; it is not very 

 profound, and is very delicately pectinated below. The 

 hinge-margin is found, when measured, to be equal to half 

 the width of the shell, though scarcely appearing so to the 

 eye. The dorsal edges are rather long and much sloping. 

 The interior is usually of a whiter cast than the external 



