LUCINA. 63 



traced out the series, and drawn up the following description 

 from the adult, in which a second lateral lamina is developed, the 

 lunule is no longer minute, and the peculiar projecting angle of 

 the margin above the front lateral tooth becomes occasionally ob- 

 solete. The shape is suborbicular, but varies in its proportions of 

 length and breadth ; the texture is strong, opaque, and of a pure 

 white externally, but frequently tinged with yellow in the inte- 

 rior, where there is never the slightest appearance (as there ordi- 

 narily is in tigerina) of crimson stains. The valves, which are 

 not very inequilateral, are rather ventricose, the chief convexity 

 being at the subumbonal region ; there is no division of surface 

 caused by any partial flattening of the lateral areas. The entire 

 exterior is covered with very numerous radiating costella?, which 

 are finest and most closely set in the middle, but become coarser 

 rather less approximate and more divergent at the sides, where 

 they are more manifestly bifurcated. This divergence is most 

 evident near the hinder dorsal surface. Both the costellaa and 

 their interstices are decussated by very delicate concentric slight- 

 ly raised striulre, which do not form regular sublimate scales on 

 the former, but simply traverse them in the most crowded man- 

 ner ; there are occasionally, however, some granular projections 

 near the front extremity. The ventral margin is much arcuated, 

 swelling the more in front, and rising the more behind, so as not 

 unfrequently to give an obliquity of outline to the shell. The 

 umbones, which incline very considerably forward, do not project 

 very greatly above the dorsal margin. The declination of the latter 

 is very trifling ; the front part, which is short and more or less in- 

 curved, terminates at the end of the sunken cordiform lunule, at 

 which point it makes a more or less marked angulation with the 

 upper and straighter part of the anterior side. The hinder 

 dorsal edge, which is much the more elevated and more or less 

 arcuated, is ordinarily, but not invariably, subangulated, yet 

 somewhat indistinctly, at its termination ; the upper part of 

 the posterior margin is also generally the straighter. Both ex- 

 tremities are rather broad, and tolerably, but not symmetrically, 

 rounded ; that of the hinder side, which is rather the longer, is 

 the more obtusely so. There is no umbonal ridge : the beaks are 

 very acute; the ligament is sunken and partially concealed. 

 The inner margin is all but entire ; the hinge-margin of the right 



