124 EOCENE MOLLUSC A. 



LEDA. Schumacher, 1817. 



Generic Character. Shell equivalve, inequilateral, elongate, lanceolate, or elliptical, 

 sometimes fig-shaped, rounded at the pedilateral margin ; siphonal region more or less 

 pointed or rostrated ; umbones small, approximate ; hinge with numerous sharp, generally 

 angular, and interlocking teeth, separated by a spoon-shaped or triangular fossette ; con- 

 nexus cartilaginous ; palleal line sinuated, with a linear impression, more or less distinguished, 

 extending from the middle of the umbonal region to the base of the oral adductor muscle- 

 mark. In the recent state the shell is covered by a thick epidermis. 



Animal with the mantle open in front, margins simple, sometimes fimbriated ; foot large, 

 discoidal, with serrated edges ; siphons slender, unequal, partially united. 



This genus is distinguished from Nucula by the prolongation or extension of the siphonal 

 region making some of the species equilateral, or even reversing the comparative dimensions, 

 giving the smaller and shorter portion to the pedal side. Moller has subdivided this 

 genus, and proposed the name of Yoldia for those shells which gape on each side. 

 These two names are still adopted by some authors, who maintain the distinction to be 

 sufficient for generic division. Morsch has made another division for those which are 

 closed at both extremities, and for which he proposes the generic name of Portlandio, 

 while Leda proper is said to gape only on one side. The distinctions here spoken of are, 

 I fear, very unstable, and will probably, by the greater number of conchologists, be considered 

 only of specific value. Solinella has a similarly shaped shell, with the dental area furnished 

 with a row of small, sharp, angular teeth on each side of the umbo similar to those of the 

 present genus ; but the connector there is wholly ligamental, situate externally upon a pro- 

 jection or fulcrum, and this character fairly entitles the shell in question to generic 

 distinction. 



The shells of this genus {Leda) are often quite smooth ; but some have concentric 

 ridges or thickened lines of growth, and a few are ornamented with ridges in an oblique 

 direction, the inner margins of the shells are at all times free from crenulations. 



Species of this genus in the recent state have been found in all parts of the world and 

 in all climates, and at all depths : upwards of seventy have been described ; and in the 

 'Proc. of the Nat. Hist. Soc. of Philadelphia,' 18G0, p. 49, Mr. Binney called attention 

 to a species of Leda which Dr. Gould says is common to the seas of Japan and to the coast 

 of Massachusetts. 



In the fossil state they are also numerous, and have been found low in the Secondary 

 formations. 



