164 BIVALVES. 



by the ligament which is internal, the lateral 

 teeth laminar, double in one valve, inserted. 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



The shells of this genus are clearly marked 

 by their angular central tooth, and internal 

 ligament. They vary little in form or colour- 

 ing, and do not display much beauty; they 

 are usually semipellucid, and of a thin delicate 

 appearance; the surface is seldom diversified 

 by inequalities. The prevailing colour is blueish, 

 or yellowish white, sometimes radiated. The 

 animal has not the power of progressive motion, 

 but by the assistance of a small compressed 

 muscular foot, it is enabled to change its posi- 

 tion, a little : this organ projects at one end of 

 the shell, and at the other are two tubes formed 

 by the mantle. The name Mactra is derived 

 from the Greek paKrpa, (mactra) a kneading- 

 trough. 



MACTRA Stultorum* 

 SIMPLETON'S KNEADING-TROUGH. 



Specific Character. Shell subtriangular, con- 

 vex, thin, brittle, semipellucid ; colour white 



* Plate VIII. fig. 4. 



