1874.] 245 [Rathbun. 



hinge and incline strongly forward. The hinge equals a little more 

 than one-half the length of the shell, and descends quite rapidly from 

 the beaks toward the posterior margin, where it ends abruptly. The 

 latter, which has about the same height as the anterior margin, be- 

 gins, in the internal moulds, at a higher level than the posterior ex- 

 tremity of the hinge, since at this point, as is generally the case in 

 Nu;ula, the cavity of the shell extends nearer to the true margin 

 than it does along the hinge. It rounds strongly to the ventral mar- 

 gin. The anterior margin curves quite rapidly downward and back- 

 ward, but is not so fully rounded as is the posterior margin. 



The valves are most convex in the lower part of the umbonal re- 

 gion, and curve moderately from the beaks to the ventral margin. 

 The curvature of the surface from the anterior to the posterior mar- 

 gin is also very moderate and quite regular, though the valves are 

 slightly flattened in the middle. By the thickening of the margins 

 of the shell, where they approached one another, there has been 

 formed a slightly flattened or concave area or band on the mould, 

 bordering its margins, which are very acute. 



Only a single specimen was obtained, the interior moulds of the 

 two valves attached. In breaking this from the rock, the exterior 

 mould was injured beyond repair, but enough remains to show that 

 the surface of the shell was smooth, or marked only with indistinct 

 concentric lines. Size : length, 3.25 mm.; height, 2.5 mm.; depth of 

 two valves, 1.5 mm. 



In its interior characters this form approaches very closely some of 

 the varieties of Nucula pectinata Sow. of the European cretaceous, 

 but the latter form is a very much larger one and the surface of the 

 shell is marked with prominent radiating lines. 



From the cretaceous limestone bed at Maria Farinha, Prov. of 

 Pernambuco, Brazil. 



Family LEDIDJE. 



Leda Swiftiana sp. nov. 



Shell very small, elongate and nioderately gibbous. It is elongate- 

 ovate in outline, with the length more than one and one-half times 

 the height. The beaks are very large, prominent and quite strongly 

 incurved; they are situated a little in advance of the middle of the 

 shell. Posterior to the beaks, the hinge descends quite rapidly to the 

 posterior margin and forms a slight outward curve. The anterior 



