154 ARCID^. 



writers to the so-called genus Yoldia, is closed at both 

 ends. These (and probably L. pernula) being the only 

 species indigenous to our seas, I must hesitate before 

 admitting the proposed division of Leda into two genera, 

 so far as concerns the British Mollusca. 



A. Shell triangularly oval, smooth and polished; posterior 

 extremity rounded and closed : lunule indistinct ; teeth set 

 like those of a comb. 



1 . Leda pygm/e'a *, Minister. 



Nucala pygmcea, (Minister) G-oldfuss, Petref. p. 157, t. cxxv. f. 17. L. 

 pygmtea, F. & H. ii. p. 230, pi. xlvii. f. 10, and (animal) pi. P. f. 3. 



Body pale fawn-white colour : tubes short, united, with plain 

 orifices : foot hatchet-shaped, and widely grooved at its poste- 

 rior edge, forming when expanded a crenated disk. 



Shell nearly equilateral, sloping gradually from the beak to 

 each side, tumid, glossy, and iridescent : sculpture, only a few 

 irregular marks of growth : colour pearl-white under the epi- 

 dermis, which is yellowish-brown, highly polished and lustrous: 

 margins rounded at the anterior side, with an oblique slope to 

 the ventral range, which is also rounded, somewhat produced 

 and wedge-shaped at the posterior side, with a slight tendency 

 to curve upwards : beaks small and blunt, rather incurved than 

 recurved ; lunule lance-shaped, but scarcely visible, and faintly 

 denned by an obscure ridge, which runs from the beak to the 

 posterior angle in each valve : ligament exceedingly slight : 

 cartilage and the cavity containing it very small and short : 

 Mage-line slightly curved, occupying nearly the whole of the 

 dorsal area and considerably more than one-third of the cir- 

 cumference : hinge-plate deeply grooved, and becoming wider 

 as the distance from the cartilage -pit increases : teeth rather 

 long and sharp, placed on the inner margin of the hinge- 

 plate, slightly curved outwards or reflected ; there are 10-12 

 on the anterior side, and 12-14 on the other : inside glossy, 

 with the margin somewhat thickened and entire : pallial and 

 muscular scars slight, except in aged or dead specimens. 

 L. 0-15. B. 0-2. 



* Dwarf. 



