NUCUI.ID.*. 



LEDA, Sc/inmaclier. 



Gen. Char. More or less elongated, usually peaked behind, almost 

 always clothed with an epidermis, not nacreous within ; margin 

 never crenated. Teeth pectinated, on both sides of an internal 

 cartilage. Pallial line sinuated. 



Sect. I. Leda, proper. — Often strong, usually with an umbo- 

 nal ridge and concentric sculpture, only gaping at the posterior 

 tip, sometimes without au epidermis, which, if present, is more 

 often unpolished. Both dorsal areas usually impressed or defined, 

 almost always sculptured. Pallial sinus small or moderate. — The 

 four final species (esjiecially the last two), Nicobarica, serotina, 

 lepidula, and lenella, are intermediate in character between this 

 and the next section. 



Sect. II. YoLDiA, Moller. — Gaping at both ends, rarely with 

 an umbonal ridge, usually fragile, usually with an epidermis, if so 

 always a polished one (except, perhaps, in Thniciaformis), never 

 ribbed, nor radiaiingly sculptured. Dorsal areas rarely, if ever, 

 sculptured, their lips usually pouting ; front dorsal area rarely im- 

 pressed or defined. Cartilage-pit usually large, cochleariforni, or 

 broadly triangular, shelvingly projecting inwards. Pallial sinus 

 usually large. 



Sect. III. PoRTLANDiA, Mi'irch. — ^Resembling the last, but 

 closed at both ends. 



Sect. I. Leda, proper. 



1. L. Ei.oNGATA, Soicb., f. 39. — T. cymbaifovmis, utrinque liians 

 et obverse subcuneiformi.-, sequilatcralis, compressa, semipellucida, 

 striis pertenuibus et antiee reraotioribus longitutlinaliter scalpta, 

 postice Icevigata, cute nitida tenuissima fulva vel olivaceo-cinerea 

 induta. Extremitas antica aliquantulum latior, magisque rotun- 

 data ; postica subbiangulata. Margo dorsalis horizontalis ; ventra- 

 lis in medio rectiusculus vel subretusus, utrinque convexiter accli- 

 vis. — Boat-shaped, thin, semitransparent, more or less compressed, 

 gaping at both ends (more so in front), white under an excessively 

 thin epidermis, which in aged examjjles is olivaceous ash-coloured, 

 but in less matured ones is shining, bright fulvous, and only 

 conspicuous near the margins ; posteriorly smooth, elsewhere 

 scratched lengthways with shelving striae, which are so fine above 

 as to appear obsolete, are close, fine, and regular in the middle, 

 more remote and concentric in front. Ho umbonal ridge, but au 

 indistinct shallow indented ray in front. Sides equal, or very 



