LAMELLIBRANCHIATA. 247 



Ear small, well-ck'tine:l in young specimens, and scarcely increasing in 

 size in proportion to the growth of the shells, limited by a distinct byssal 

 depression; extremity rounded. Wing large, triangular ; margin more or less 

 concave in young shells, aud often nearly straight in older shells ; extremity 

 produced or rectangular, often extending beyond the posterior margin of the 

 valve. 



Test thick, especially in the umbonal region ; marked by distinct sub- 

 equidistant, rounded railii, with slightly concave interspaces, in which are 

 often finer, or obscurely developed rays ; the right valve is usually more 

 closely and less strongly marked by them than the left. The radii are 

 crossed and crenulated by elevated, lamellose, concentric strife, which are 

 more distant and conspicuous on the right valve. In the 3^oung shells, the 

 radii are conspicuous on the wing; they are a little more distant than on 

 the body of the shell, and often ol^solete along the line of the junction of the 

 wing with the body, especially in the right valve. In both valves, the radii 

 become obscure or obsolete below the centre, and the surface is marked by 

 strong, irregular, concentric, lamellose undulations. The radii often become 

 more or less obsolete upon the wing, while remaining conspicuous on the 

 body of the shell. 



Ligamental area extending the entire length of the hinge-line, much wider 

 in the left valve ; marked by coarse parallel, undulating, longitudinal grooves 

 or strise. There is a conspicuous oblique posterior lateral tooth, and two 

 strong cardinal folds beneath or just anterior to the beak. Anterior 

 muscular impression situated in the rostral cavity ; the pallial line continues 

 thence in a curve somewhat parallel to the margins of the shell, turning 

 inward below the middle of the height and joining the large muscular 

 impression. 



The right valve of a young individual (plate xxiv, fig. 14) has a length 

 of 30 mm., height 35 nmi., and hinge-line 32 mm. The margin of the left 

 valve, which remains in the matrix, shows it to have had a length of 42 mm., 

 and a height of 53 mm. Another specimen, with the two valves attached, 

 has the left valve 75 mm. and the right valve 55 mm. in height, while the 



