32 CONCHOLOGY. 



umbones decorticated; internal disc of the valves white, 

 thick, and callous. 



FAMILY IV. 



Myaria. Two genera. 



1. Mya. The Trough Shell, or Gaper. Four species. 



This term is derived from the Greek word fivu, to close, 

 alluding to the animal's custom of closing the valves. The 

 principal characteristic of the Mya is its gaping at one end ; 

 it is likewise distinguished by having a large spoonlike 

 tooth proceeding from beneath the beak. Its form is greatly 

 varied, but generally covered with a greenish epidermis, 

 which may be removed ; and the shell, when polished, will 

 display beautiful prismatic colours. The Mya is found on 

 the seashore or on the banks of large rivers, partially con- 

 cealed in the sand and mud. 



Shell transverse, inequilateral, surrounded with a thick 

 epidermis ; rather solid ; edges thin and sharp ; summits but 

 little marked ; hinge dissimilar ; one or two large, compress- 

 ed, spoon-shaped teeth rising perpendicularly from the plane 

 of the left valve, and fitting into the entrance of a primary 

 cavity in the right valve ; ligament interior, attaching the 

 tooth and cavity ; two distant muscular impressions ; the 

 anterior long and narrow, the posterior rounded ; the mantle 

 impression narrow, with a large sinus or hollow. 

 Mya truncata. Mya erodona. 



M. arenaria. M. solenimyahs. 



M. arenaria. The Sand Mya. PI. 5, fig. 1. 

 Regular species. 



M- erodona. The Erodona Mya. 



Irregular species, in which the cavity of the right valve 

 is bordered by strong projections. 



M. truncata. The truncated Mya. PI. 5, fig. 3. 

 Sub-oval, truncated ; small end gaping ; large end round- 



