58 



whether or no it inhabits the ocean, I am unable to de- 

 cide. I once found it among the fossils, exclusively ma- 

 rine, of the Newer Pliocene in Maryland, but it was 

 quite rare. The living shell, however, prefers the 

 brackish waters of bays and lagoons, where it congre- 

 gates in vast numbers. The extensive flats below Mo- 

 bile, Alabama, are literally filled with them, where they 

 burrow about three inches below the surface, and give 

 token of their presence by a rather obscure depres- 

 sion in the sand. They are accompanied by only two 

 other species of shells, the Cyrena CaroUnensis and 

 JVeritina reclivata, of Say. Some idea of the for- 

 mer abundance of the Rangia may be obtained, when 

 it is stated that the streets of Mobile are paved with 

 them in a subfossilzed state, and that the shore on 

 which the city is built, and the marshy islands of the 

 bay, have at various depths below the surface, a solid 

 stratum of shells, the depth of which is unknown to 

 me. They are said to be equally abundant on the 

 borders of all the lagoons and bays which margin or 

 indent the northern shore of the Gulf of Mexico. 



MACTRA. 



GENERIC CHARACTER. 



Shell inequilateral, subtrigonal, slightly gaping at 

 the posterior end ; beaks prominent, with a com- 

 pressed complicated cardinal tooth in each valve, and 



