20 University of Michigan 



Actinonaias (Disconaias) walked, new species 



Plate I, figs. I and 2; plate IX, fig. 49, type; plate X, figs. 48-50; 

 plate XI, figs. 48 and 49 



Fifteen specimens, including odd valves, from Rio San Juan 

 (H, vii, c). 



Shell rather large; male(?) subrhomboid, elongate, com- 

 pressed, often slightly sinuate below; female(?) subelliptical, 

 rather swollen ; anterior end rounded, with dorsal margin con- 

 siderably lower than beaks and the ventral margin obliquely 

 truncate; posterior end with dorsal margin equal (males?) or 

 higher (females?) than the beaks; posterior ridge double, 

 rounded to subangular in the males( ?), while the region is con- 

 siderably swollen in females ( ?) ; posterior margin moderately 

 biangulate ; beaks rather low, behind the middle, and curved 

 posteriad, especially in the females( ?) ; sculpture (if identi- 

 fication of juvenile specimens is correct) probably consisting 

 of fine, concentric wrinkles, flattened ventrad ; epidermis rough- 

 ened by irregular growth-lines which are sometimes markedly 

 sulcate, especially near the ends of the shell, radially striate 

 throughout and sometimes with dorsally curved sulcations on 

 the anterior slope ; color brownish with very indistinct olive- 

 green rays in adult (rays diffuse on a yellowish background 

 in juvenile shells) ; left valve with two stout, trigonal, jagged 

 pseudocardinals, and two heavy, short laterals (fig. 49, plate 

 XI) ; right valve with two pseudocardinals, the anterior small 

 to vestigial, oblique; the posterior large, stout, trigonal, almost 

 vertical, and broken superficially by vertical, jagged lamella- 

 tions ; with cavity for reception of posterior left pseudocardi- 

 nal, usually deep and large ; followed by a small tooth ; and 

 with one very heavy, club-shaped, short lateral ; hinge-plate 

 heavy, short, occupying middle half of dorsal margin, curved 

 below in male( ?) and almost arcuate in female ( ?) ; beak cavi- 



