gt2 QUADRULA 
QUADRULA PERCOMPRESSA (von Martens). 
Shell large. triangular or subtriangular, solid, greatly com- 
pressed, with very high beaks, whose sculpture has not been 
examined; equilateral or inequilateral; posterior ridge nar- 
rowly biangulate ; anterior slope often incurved above, rounded 
below ; posterior slope rounded ; surface with strong, nodulous, 
concentric ridges; in some cases the ridges become almost re- 
placed by nodules; epidermis pale brownish, the surface gen- 
erally nearly decorticated ; left valve with two low, solid, radial, 
rough pseudocardinals and one lateral with a vestige of a sec- 
ond above it; right valve with a large, somewhat divided, pseu- 
docardinal and one lateral; hinge plate very wide and flat and 
on it there is a wide epidermal deposit under the beaks, which 
often covers the entire width of the plate; beak cavities very 
deep and compressed ; anterior scars shallow, rough; posterior 
scars rather long ; nacre varying from whitish to purple, great- 
ly thickened in front; pallial line remote from the shell border 
in front. 
Length rto, height 96, diam. 25 mm. 
Leneth 94, height 92, diam. 28.5 mm. 
Length 95, height 73, diam. 24 mm. 
Rio Usumacinta and Rio de las Salinas, Guatemala. 
Unio percompressiis VON MARTENS, Sitzungs Ber. Nat. Tr. 
1887, p. 107; Biol. Cent. Am. Moll., p. 406, pl. xxxm, figs. 
1-3. 
Ouadrula percompressa SIMPSON, Syn., 1900, p. 796. 
A remarkable shell, of which two large, teft valves are in 
the National Museum, both of which turn to the left in front 
and to the right behind. From an examination of these re- 
markable valves I believed it best to give the species the rank 
of a group, but the figures, which von Martens has lately pub- 
lished in the Biologia, show much variation in the species and 
that it varies towards QO. quadrata and ostreata. I therefore 
place it in the same group with these species. It cannot be 
mistaken for any known form. 
