81 



greater than the length. The shape varies in different individuals, and 

 the variation of the contour fan be best expressed by a separate 

 descrijition of eaoli part. 



The hinge line is usuall}' straight and oblique, but the amount of 

 obliquity is greater in some specimens than in others. In fig. A, the 

 maximum of obliquity is i-eached ; in iigs. C and D, the hinge line is 

 more nearh- horizontal; and in fig. B, it was probably a little convex. 

 There are between ten nnd twelve cartilage pits in the hinge, and these 

 sometimes, though not always, are narrowest at the postei'ior end. 



The beaks are slender, elongated and tei-minal ; they are unequal in 

 size and of a different shape, but both project considerably bej'ond the 

 anterior margin. As viewed from above, the beak of the right valve 

 curves decidedly outwards from the hinge line at a short distance from 

 the apex, and then turns rather suddeid}' in\vards anil almost backwards. 

 The beak of the left valve is straight, and points forwards, the extreme 

 tip being bent slightly downwards. The anterior margin is usuall}^ 

 shallowly concave from the beaks to about the middle, so that the tw^o 

 sides of the upjier half of the shell are almost exactly parallel. In 

 some specimens, the concavity of the margin in the region of the byssal 

 opening docs not quite extend to the middle. Beneath the centre the 

 anterior border generally widens a little, and becomes gently convex, 

 but occasional!}^ it is almost straight. The front margin, as a whole, 

 has the shape of a sigmoid curve, but this is sometimes almost 

 straightened out. In some examples, the shell is as wide or wider 

 below than above ; in others it is narrowest below. 



The bj'ssal orifice is well shown in two or three instances. Below 

 the beaks, in front, the edge of the left valve projects slightly beyond 

 that of the right, and between both there is a long but ver}- nai'i-OAv 

 opening. 



The shape of the posterior mai-gin is very variable in different 

 specimens. In some it is almost straight and perpendicular; in others, 

 it is rather oblique above the middle, and gradually widens and becomes 

 decidedly convex below; but in every case it forms an angular junction 

 with the hinge line above. The base is either evenly and narrowly 

 rounded, or else it is produced and somewhat pointed in front of the 

 middle. ' 



The surface is marked apparenth' by irregular and, for the most part 

 distant, concentric stria', but the test is exfoliated in all but a single 

 specimen, and on this there is only a small portion of the outer layer 

 preserved. 



