2:20 MALACOZOA. TROPIOrODA. LAMELLIBRAXCHIATA. 



1. 3/yflf fruncata. — Shell ovate, abruptly truncate 

 behind. Trunratus, cut short at one end. 



2. Mya arenaria. — Shell ovate, rounded at both ends. 

 Arenarius, linng among sand. 



3. Ml/ a ovalis. — Shell ovato-elliptieal, rounded and 

 somewhat reflexed behind, the umbones nearly central. 

 OvdliSi like an egg. 



Genus 2. Ltonsia. — Shell oblong, much compressed, 

 inequivalve, inequilateral, open behind ; umbones very 

 small ; hinge toothless, but with an intervening calca- 

 reous body. Xamed after ]Mr. Lyons. Turton. 



1. Lijonsia Xonccgica. — Shell oblong, very thin, 

 pearly within, the anterior end shorter and rounded, the 

 posterior truncate. Xamed 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. S^j/i', a wedge. Turton. 



1. Sphi'nia Swainsdnii. — Shell oblong, rounded at both 

 ends, much compressed, deeply striated concentrically. 

 Named after 'Six. Swainson. 



2. Sphenia costiddta. — Shell ovato-elliptical, mode- 

 rately convex, radiatingly costulate, concentrically striu- 

 late. Cost ul at us, having small ribs. 



Genus 4. Moxtacuta. — Shell ovate or oblong, rounded 

 at both ends, equivalve, inequilateral, slightly gaping 

 behind, very thin, fi-agile, 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 hidentata. — Shell ovate, somewhat pel- 

 lucid, very inequilateral, the posterior end much shorter 

 and narrower. Bidentdtus, having two teeth. 



2. Montacuta ohJunga. — Shell oblong, very thin, 

 compressed, transparent, inequilateral, rounded at both 

 ends. Ohlongusy longer than broad. 



3. Montacuta glabra. — Shell ovato-oblong, sub- 



