its posterior line transverse, slightly arquated, widest : 

 within white, perlaceous, iridescent before : teeth large, 

 prominent, the cardinal anterior of the right valve and hoth 

 of the left valve parallel to the lateral teeth, which are a 

 little arquated, those of the right valve united at tip and 

 even after the union a little more elevated than the sur- 

 rounding surface : sinus of the cartilage, regular, definite : 

 cavity of the umbo not deep, its muscular impressions ve- 

 ry distinct : posterior muscular impression remarkably 

 deep, subobconic : palleal impression anteriorly arquated, 

 hardly extending beyond the line of the muscular impres- 

 sion. 



OBSERVATIONS. 



This species is not uncommon. The young shell which 

 we have represented in the plate has a different colouring 

 from the old specimen, which is of a uniform dark, or 

 even blackish-brown, the radii being entirely invisible. It 

 varies in its radii, some being radiated over the whole sur- 

 face, others having capillary radii only on the anterior side 

 and others being entirely destitute of radii at all ages. An- 

 other variety is tinged with rosaceous, especially on the 

 hinge laminae. In its youth it somewhat resembles the 

 sulcatus. Lea, but is distinguished by many characters, and 

 remarkably by being destitute of any sulcation anteri- 

 orly. The largest one I have seen was nearly four inches 

 in its greatest diameter ; but this is gigantic for the species. 



REFERENCE TO THE PLATE. 



Upper figure — exterior of a valve. 

 Middle figure — back view of the eholi. 

 Lower figure — inside of a valve. 

 PLATE XIY. 



