TAPES. 393 



and rather glossy. The valves are more or less ventricose, 

 the umbones being disposed to tumidity ; and their surface 

 is concentrically traversed by close-set not strictly parallel 

 narrow sulci, the interstices of which are often bifurcated, 

 and not unfrequently become obtuse costellaj, especially in 

 front and towards the lower margin. This sculpture, 

 which has a tendency to become obsolete upon the most 

 swollen portion of the shell, the hinder part of the um- 

 bonal region, is indistinctly decussated by impressed radi- 

 ating lineoles. The ground-colour, which is usually of a 

 pale golden yellow, or creamy hue, often whitish, is most 

 frequently marbled with linear or cloudy zigzags of many 

 shades of liver and smoke-colour, differing in various ex- 

 amples in respect to the size and propinquity of mark- 

 ings. More rarely, the shell is perfectly devoid of any 

 variegation, and is of an uniform pale yellow, or white. 

 The ventral margin is always more or less subarcuated, 

 and is usually a little contracted posteriorly in the adult. 

 The declination of the front dorsal edge is retuse, and 

 always more or less strong ; that of the hinder one is con- 

 siderably less, and convex, though frequently but slightly 

 so. The anterior side, which occupies from one-third to 

 two-fifths of the entire length, is well rounded at its 

 extremity, which from its greater or lesser attenuation 

 often appears strikingly projecting. The posterior termi- 

 nation is very variable in its contour ; in the younger shell, 

 it is generally more or less bluntly and obliquely subbian- 

 gulated ; in the adult it is more nearly rounded, and often- 

 times is produced attenuated and thoroughly rounded. 

 The lunule is very large, ovately lanceolate, not profound, 

 but defined by a shallow line. There is no posterior 

 escutcheon. The umbones are more or less prominent, and 

 the beaks are very acute, small, and inclined forwards. 



VOL. I. 3 E 



