CONCtiYLlA—DITHYRA. 4. 19 



Shell half an inch long, and nearly an inch broad, oval, wedge- 

 shaped, thin and brittle, but not transparent, white or covered 

 with a blackish coat, not glossy, very open in front, with the 

 edges of the margin thin and sharp, irregularly wrinkled trans- 

 versely : beaks rather prominent and obtuse, placed near the 

 larger end, where the valves meet close together at the back to 

 the extremity of the shell : hinge with an obscure laminar trans- 

 verse tooth in each valve. 



A variety of about half the size, but in no other respect differ- 

 ent is found buried in the back of old oyster-shells. 



4. ^XXICA\ A— LAMARCK. 



Testa transversa, (squivalvis, incBquilaieralis, hinc hians. 



Cardo edenttdus. Ligamentuni externum. 

 Shell transverse, equivalve, inequilateral, open at one or both 



ends. Hinge without teeth. Ligament external. 



This, and some of the subsequent genera, from their inhabiting 

 the interior of rocks, and their attachment to marine substances' 

 seem naturally allied to the Pholas tribe, and to each other. The 

 individuals of this genus are so variable and irregular in their 

 shape, that it is extremely difficult to reduce them to distinct and 



I) 2 



