44 CONCHY LI A— DJTHYR J. 14. 



pr rather transverse position; and others projecting inwards in 

 an horizontal direction, having an independent marginal circum- 

 ference nearly all round. 



Many of them lie buried deep under the sand, and rarely come 

 to us in a living state; others inhabil the interior of sub-marine 

 rocks, from which we have taken them alive. 



All of them have a rough shagreen-like surface; and they are 

 generally of an uniform dead white color, occasionally mixed with 

 an ochraceous or muddy contamination. 



The ligament in this and some few other genera, is double ; a 

 short external one, and an internal one in the hollow of the spoon- 

 shaped teeth. But in conformity with our general arrangement, 

 Ave consider the ligament to be generically external, when it is 

 externally visible. 



/ l.amarck has made the oblique internal rib, which proceeds 

 from the insertion of the teeth on the anterior side, an essential 

 part of his generic character. This however is by no means a 

 constant mark, and apparent only in such as have that side more 

 strongly angulatcd. 



From the genus Listera it differs, in having no distinct teeth 

 in addition to the spoon-sliaped ones; and from Amphidesma, in 

 wantina: the lateral tcclli. 



coHYexa. 

 1. 



'» 



* Dentibus transversim obliquis. 1-3. 

 "With the teeth transversely oblique. 



An ATTN A testd convexa ovato-ohlongd, latere antico aiigulato 

 subtruncato, dentibus angustis mtegerrimis. 



