LEDA DECUSSATA. 57 



Shell ovately triangular, inflated in front, behind attenuate, rostrate : um- 

 bones nearly mesial, small, incurved, contiguous ; lunule sharply elliptical, 

 obscurely margined ; area deeply excavated, bounded by a rounded, broad, 

 elevated ridge ; angle of hinge line obtuse : surface engraved with crowded 

 regular concenti'ic striae decussated by minute radial stria), and presenting a 

 beautiful appearance imder the lens. 



Three specimens. Dimensions of that which is in best condition, and 

 figured: length, 12 mm.; height, 8 mm.; thickness, 6^ mm. 



This species in form and superficial markings so closely resembles L. ve- 

 niisia Sauvage and Rigaux (Journal de Conchyliologie, XIX, p. 356, 1871 ; 

 XX, p. 180, PI. xi, fig. 7, 1872), from the Kimmcridgien moi/en de Brtqiierccqites, 

 that in the outset I was strongly inclined to look upon it as a variety of 

 vcnusta. But on further examination it seems necessary to set it apart as a 

 distinct species, on account of the following differences. The French speci- 

 mens are smaller, their three dimensions being given respectively as 7, 4, and 

 3 mm. ; they have the anterior margin rounded, and the surface of the sides 

 radiatim eostcllata d conceidrtcc striata (op. cit., XIX, p. 350), or, oruec de cotes 

 rajjonnai'des nombreiises et de strics conceniriqnes (op. cit., XX, p. 181). The Leba- 

 non shells, on the other hand, have their dimensions about twice greater than 

 those o^ veiiiisfa : viz. 12, 8, and 6^ mm. ; the front margin slopes in a nearly 

 straight line ; and the surface is engraved with close-set, well-impressed con- 

 centric striae, crossed, not by costce nor codelice, but by radial stria3 so deli- 

 cate as to be visible only with aid of the lens. So also the Jurassic L. luciy/ina 

 Sow. sp., has very nearly the form of this species, but is distinguished from it 

 in being smaller, more prolonged posteriorly into a narrow rostrum, and in 

 having its surface marked with " concentric striations, rather remote, and 

 faintly impressed, sometimes undistinguishable " (Morris and Lycett, MoUusca 

 from Great Oolite, Part II, p. 53, PI. v, figs. 15, 15 a, 1853). Coll. Bird. 



Locality and Position. — Fraas records from the Ornati Claj' of Mejdel esh 

 Shems, under the name of Nucula laciyjma Sow. (Jahrbuch fur Mineralogie, 

 etc., 1877, p. 27, and Aus dem Orient, II. Tlieil, p. 10, 1878), a species which 

 in all probability is the one here described. The general similarity of the 

 two species has already been noticed above. One of our three specimens 

 has the test partly broken away, showing within a cast having the pyritif- 

 erous composition mentioned in connection with the species last described. 

 For reasons there and here stated, it is probable that these specimens of 

 Leda ai'e from tlu' Ornati Clay of Mejdel esh Shems. 



8 



