220 MALACOZOA. TROPIOrODA. LAMELLIBRANCHIATA. 



1. My a truncdta. Shell ovate, abruptly truncate 

 behind. Truncdtus, cut short at one end. 



2. My a arendria. Shell ovate, rounded at both ends. 

 ArendriuSy living among sand. 



3. My a ovdlis. Shell ovato-elliptical, rounded and 

 somewhat reflexed behind, the umbones nearly central. 

 Ovdlis, like an egg. 



GENUS 2. LYONSIA. Shell oblong, much compressed, 

 inequivalve, inequilateral, open behind ; umbones very 

 small ; hinge toothless, but with an intervening calca- 

 reous body. Named after Mr. Lyons. Turton. 



1. Lydnsia Norweyica. Shell oblong, very thin, 

 pearly within, the anterior end shorter and rounded, the 

 posterior truncate. Named from its occurring in Norway. 



GENUS 3. SFHENIA. Shell oblong, inequivalve, inequi- 

 lateral, gaping behind ; umbones very small ; hinge of 

 the left valve with a thin dilated horizontal adnate tooth, 

 that of the right valve with a concave tooth and an an- 

 terior minute prominence. 2^17^, a wedge. Turton. 



1 . Sphenia Swainsdnii. Shell oblong, rounded at both 

 ends, much compressed, deeply striated concentrically. 

 Named after Mr. Swainson. 



2. Sphenia costuldta. Shell ovato-elliptical, mode- 

 rately convex, radiatingly costulate, concentrically striu- 

 late. Costuldtus, having small ribs. 



GENUS 4. MONTACUTA. Shell ovate or oblong, rounded 

 at both ends, equivalve, inequilateral, slightly gaping 

 behind, very thin, fragile, glossy ; umbones small ; 

 hinge with a deep sinus and two prominent diverging 

 teeth, one of which is concave, for the ligament. Named 

 after Montagu, by Dr. Turton. 



1 . Montacuta bidentdta. Shell ovate, somewhat pel- 

 lucid, very inequilateral, the posterior end much shorter 

 and narrower. Bidentdtus, having two teeth. 



2. Montacuta obUnga. Shell oblong, very thin, 

 compressed, transparent, inequilateral, rounded at both 

 ends. ObUngus, longer than broad. 



3. Montacuta gldbra. Shell ovato-oblong, sub- 



