87 



MYA? gregarea. 

 TAB. CCCLXIII. 



Spec. Char. Obovate, convex, subdeltoid, 

 smooth ; anterior side truncated ; lower 

 margin of the left valve produced. 



J HE anterior side of the right valve is rather cuneiform 

 and truncated ; the left valve is only truncated ; it is 

 larger all round than the right, so that it receives the 

 upper edges of that valve upon the thickened parts on 

 each side of the hinge : on the posterior side it has a 

 remote tooth, that with its accompanying hollow forms a 

 secure lodgment for the corresponding edge of the ri^ht 

 valve : within the anterior edge it has only a slight fur- 

 row : the outer surface is nearly smooth, except a few 

 lines of growth and the eroded beaks. The line upon 

 the inner surface to which the mantle of the animal was 

 united, has a small sinus equal only to the quarter of a 

 circle, placed close to the anterior muscular impression. 

 It is highly probable that this shell will become the 

 foundation of a new genus, to which will be associated 

 Myae labiata (Linn. Trans. Vol. X. p. 326, t. 21, f. 1, 2, 

 and 3,) plana and subangulata (Min. Conch, t. 76, figs. 

 2 and 3.) The former of these is a fresh-water shell, 

 inhabiting the Rio de la Plata, a circumstance strongly 

 confirmatory of the fresh-water origin of the stratum 

 near the top of Headon Hill, in which immense numbers 

 of the species now before us are closely grouped together 

 unmixed with other shells. The remote tooth, the small 

 sinus in the mantle, and the inequality of the valves, are 

 among the characters that distinguish it from Mya ; the 

 inequality of the valves, however, upon which much 



