450 Trans. Acad. iSci. of St. Louis. 



mens and is more acuminate posteriorly. This whole group 

 of species are intimately related one to another and they need 

 further study to determine their true relationships. This can 

 only be accomplished with the aid of much more extensive 

 collections than are at present available, but when it is done 

 it is possible that some of the species mentioned will prove to 

 be synonyms. 



MaCRODON 8ULCATU8 n. Sp. 



Plate2,fig$. 6-9. 



Description. Shell equivalved, of medium size, subovate 

 to subelliptical in outline, widest posteriorly, width one-half 

 or a little more than one-half the length, beaks situated an- 

 teriorly but not terminal, prominent, elevated above the 

 hinge-line; hinge-line three-fifths to three-fourths the total 

 length of the shell. Dorsal margin straight along the hinge- 

 line, obtusely subangular at each end where it joines the an- 

 terior and posterior margins; posterior margin broadly 

 rounded, sometimes obliquely subtruncate above, the greatest 

 posterior extension of the shell below the middle ; ventral 

 margin usually straight through the greater portion of its 

 length, curving upward in front and behind, sometimes 

 slightly convex throughout; anterior margin short, regularly 

 rounded. Valves gibbous in the umbonal region, the um- 

 bonal ridge merging into the general convexity of the valve 

 posteriorly ; the cardinal slope from the umbonal ridge con- 

 cave, very abrupt near the beak, becoming more gentle, pos- 

 teriorly ; the ventral slope longer and more gentle than the 

 dorsal, with a broad flattened area or a broad, shallow sinus 

 extending obliquely from the umbo to the middle of the ven- 

 tral margin. Surface marked by regular, concentric lines 

 separated along the posterior half of the umbonal ridge by 

 intervals of one-half to one milleraeter, or occasionally by 

 wider intervals, and towards the beak by smaller intervals; 

 also marked in unworn specimens, upon the posterior half of 

 the shell and especially on the cardinal slope, with very fine, 

 radiating, costae which are interrupted at the concentric 

 lines. Hinge straight, with two or three small oblique teeth 

 anterior to the beak, and one or two posterior teeth subpar- 



