LAMELLIBRANCHIA TA . 
247 
Ear small, well-delined 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, and 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 radii, 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 striae, which are 
more distant and conspicuous on the right valve. In the young shells, the 
radii are conspicuous on the wing; they are a little more distant than on 
the body of the shell, and often obsolete 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 striae. 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 mm., 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 
