LAMELLIBRANCHIA TA . 
43 
limits not strongly defined; margin straight or slightly concave; extremity 
acute-angular; in older shells comparatively narrow, with an obtuse-angular 
extremity. Anterior ear of the right valve defined by a sulcus, extending 
to the extremity of the beak; margin convex; extremity rounded. Byssal 
sinus scarcely indicated on the left valve, forming a deep, angular notch in 
the right valve. Posterior ear large, broad-triangular; length about two- 
thirds of the hinge-line; margin concave; extremity acute or right-angled 
in small specimens, obtuse in larger individuals; limits not defined. 
Test thin, thickness nearly .3 mm., ornamented by about 80 rounded 
radii, with broader concave interspaces, crossed by regular, sharp, elevated, 
continuous, concentric, foliate expansions of the test, reaching an elevation 
of about .6 mm. above the general surface. Usually the test shows only the 
strong rays, with occasionally a reticulation (plate 4, fig. 8), where the con¬ 
centric folds are only partially preserved. The figure cited represents also 
the internal mould where the test has been wholly removed. 
Ligamental area, in young shells, represented by a slight inflection of the 
margin of the hinge with a very small longitudinal groove. In more mature 
specimens this area is wider, the number and strength of the grooves 
increasing, until they occupy a continuous space between the extremities of 
the hinge-line, slightly bending outwards at the beak; the inner ridges and 
grooves do not extend the whole length of the area. Cartilage-pit and 
muscular impressions not observed. 
The original specimen of this species is 38 mm. high, 39 mm. long, hinge¬ 
line about 30 mm. Another more nearly entire specimen is 35 mm. high, 
37 mm. long, hinge-line 30 mm. The largest specimen observed shows a 
height of 78 mm., with length nearly equal, and hinge-line 42 mm. In this 
specimen the longest diameter is 85 mm., measured from a point on the 
upper anterior margin to the lower posterior margin. 
This species is distinguished by its form and surface markings. The 
characters of surface distinguish it from L. Priamus and L. macrodontus, though 
in form it bears some resemblance to the former ; and in the ligamental grooves 
it is similar to the latter. 
