20 University of Michigan 
Actinonaias (Disconaias) walkeri, new species 
Plate I, figs. 1 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 
CE vite): 
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- . 
