CO'^CEYLl \—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 tr?nsversely : 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 transverse tooth in each valve. 



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

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



4. SAXICAX A— LAMARCK. 



Testa transversa, eequivalvis, inoequilateralis, hinc Mans. Cardo eden- 



tulus. Ligamentum 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 



d2 



