LAMELLIBRANCHIATA. 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. 



