34 

 T^IACTRA. 



Truncata. jNfactra subtmncata. Don. Ei\ shells, iv. t. \16. 



Shell triangular, extremely ftrong, thick, and nearly 

 fmooth, but ufually marked with a few antiquated furrows • 

 umbo central, large, and very prominent ; the fides equal, 

 ' much flattened, and truncated ; the front margin rounded: 



hinge remarkably ftrong, which with the teeth :;re fimilar to, 

 but llronger than thofe of MaSlraJolida : the cicatriada is 

 broader than in that lpecie5?, and does not run fo far into the 

 fliell : the colour is yellowifh-white. Lengtli an inch and a 

 half; breadth a quarter of an inch more. 



This fhell which has been confounded with the folida and 

 fubtruncata, is extremely common on the fliores of the Frith 

 of Forth, in Scotland, where the fo/ida is not frequent, and of 

 infignificant growth. It difiers from that fpecies in being 

 much more ponderous, in the fides being flattened like the 

 fubtruncata, and in being more triangular, the length nearly 

 equaling the breadth ; whereas the folida of the fame breadth 

 as this, is half an inch lefs in length. In the external ap- 

 pearance it is generally much fmoother, the wrinkles not fo 

 ftrong, nor fo numerous ; the valves are alfo much deeper ; 

 in this refpedl it is more nearly allied to fubtruncata, but be- 

 fides that fliell being generally more or lefs inequilateral, 

 the fine and regular tranfverfey?/7Vp witli which it is wrought, 

 at once diflinguiflics it from this, Thefe three fpecies in- 

 variably 



