CONCHOLOGY. 35 



Others at the anterior only. The most general colour is 

 lilach, or white tinged with blue or yellow ; some have purple 

 rays on a brown ground. 



A singularity in the form of the hinge of the Mactra dis- 

 tinguishes it from all other genera. It is of a triangular form, 

 has a bent or angular compressed tooth on each valve, with 

 a small oblique cavity on each side to which the ligament is 

 attached. There are also two lateral teeth, one near the 

 ligament and the other near the primary tooth. These teeth 

 are thin and fragile ; the primary tooth is sometimes indis- 

 tinct, but the lateral teeth always exist. 



The Mactra is found buried in the sand at a little distance 

 from the seashore. Shell generally thin and brittle, covered 

 with epidermis, of a triangular form, transverse, equivalve, 

 inequilateral ; beaks prominent ; one compressed, folded, car- 

 dinal tooth, with an adjoining pit in each valve, projecting 

 inward ; lateral teeth thin, lamellous, entering, placed near 

 the hinge ; exterior ligament small ; an interior ligament 

 inserted in the cardinal pits ; two muscular impressions, 

 united by a narrow marginal tongue. 



