164 BIVALVES. 



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 colouring, and do not 

 display much beauty ; they are usually semi- 

 pellucid, 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 as- 

 sistance of a small compressed muscular foot, it is 

 enabled to change its position, 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 

 a, (mactra) a kneading-trough. 



MACTRA StuLiorum* 

 SIMPLETON'S KNEADING-TROUGH. 



Specific Character. Shell subtriangular, con- 

 vex, thin, brittle, semipellucid ; colour white 



* Plate VIII. figure 4. 



