44 CONCHYLIA— D/THFi?^. 14. 



or rather transverse position ; and others projecting inwards in an 

 horizontal direction, having an independent marginal circumference 

 nearly all round. 



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

 us in a living state ; others inhabit 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 

 au 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, we 

 consider the ligament to be generically external, when it is exter- 

 nally visible. 



Lamarck 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 ap:.an'nt only in such as have that side more strongly 

 an^^ulated. 



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

 addition to thi' spoon-shaped ones ; and from Amphidesma, in want- 

 in tr the hiteral teeth. 



'•* Dentihus transversim obliquis. 1-3. 

 With the teeth transversely oblique. 



conyexa. ^"^ ATT>'A te.sid convexd ovato-ohhvgd, latere antico angulato subtrun- 



1, 



!UQ, dentihus angustis integerrimis. 



