278 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



end ; in the other valve a medial impression, and two 

 lamelliform lateral teeth, with a parallel groove and pro- 

 minent margin. Ligament internal. Muscular im- 

 pressions moderate, remote. 



1. Lepton nitidum. Shining Lepton. 



Shell ovato-orbiciilar, subtrigonal, moderately compressed, 

 neai'ly equilateral, slightly broader at one end, more rounded 

 at the other ; the umbones very small, pointed ; the valves 

 thin, semitransparent, externally glossy, faintly concentrically 

 striate, internally shining. Length a twelfth and a-half, height 

 nearly a fourth less. 



Found by me, in September, 1842, among shell sand, from 

 the beach between the mouths of the Dee and the Don. 



Lepton nitidum. Turt. Brit. Biv. 63. — Lepton nitidum. Flem. 

 Brit. Anini. 429. 



Genus 3. Cryptodon. 



Shell suborbicular, nearly equivalve, inequilateral, very 

 thin, concentrically striated. Umbones small, oblique, 

 rather obtuse. Hinge extremely narrow, with a small 

 tooth in each valve, and thickened margins. Two lon- 

 gitudinal plicce along the posterior margin. Muscular 

 impressions small and submarginal. Ligament external. 



1. Cryptodon fie xuosus. Flexnous Cryptodon. 



Shell suborbicular, very convex, thin, brittle, transparent, 

 with rather distinct irregular concentric striae ; a deep sinus or 

 wide groove bounded by two rounded plicae from the umbo of 

 each valve along its posterior margin, which is obtusely angu- 

 late ; an ovate impression before the umbones ; a single incon- 

 spicuous tooth in each valve, without lateral teeth, the margins 

 being merely thickened ; the muscular impressions distant ; 

 the inner surface glossy, faintly striate ; the outer less glossy ; 

 the colour white. Length four-twelfths, height four-twelfths 

 and a-fourth. 



Brought up from deep water, off Aberdeen, frequently ad- 

 hering to Ascidias and especially Actiniae. 



Cryptodon flexuosus. Turt. Brit. Biv. 121. PI. 7. f. 9, 10.— 

 Tellina flexuosa. Mont. Test. Brit. 72. — Lueina sinuata. Lamk. 

 Svst. v. 543; Ed. 2. vi. 230. — Amphidesnia flexuosa. Lamk. Svst. 

 v.* 492; Ed. 2. vi. 128. 



