87 
MYA? gregarea. 
TAB. CCCLXIII. 
Spec. Char. Obovate, convex, subdeltoid, 
smooth ; anterior side truncated ; lower 
margin of the left valve produced. 
The anterior side of the risdit 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 right 
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. 24, 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 
