220 MALACOZOA. TROPIOFODA. 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. 

 Arendrius, 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 Norwegica. — 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. Zcfrrjv, a wedge. Turton. 



1 . Sphenia Sivainsdnii. — 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. Costiddtus, 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 glabra. — Shell ovato-oblong, sub- 



