124 EOCENE MOLLUSCA. 
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 Wucu/a 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. Méller 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. Mérsch has made another division for those which are 
closed at both extremities, and for which he proposes the generic name of Porélandia, 
while Zeda proper is said to gape only onone 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. So/¢nel/a has a similarly shaped shell, with the dental area furnished 
with arow 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 fulerum, 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,’ 1860, 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. 
