UNIO 715 
rather fine, irregularly concentric ridges, having a tendency to 
fall into two loops; the whole surface strongly and closely con- 
centrically ridged; pseudocardinals compressed, high, slightly 
curved upward; laterals compressed, curved; beak cavities 
moderately deep; nacre whitish; anterior muscle scars deep, 
rough ; posterior well marked. 
Animal with the marsupium pad-like, occupying all but the 
extreme posterior part of the outer gills; branchiz elongated, 
wider behind, inner the larger throughout, free from the ab- 
dominal sac or united to it; palp1 large, rounded behind ; man- 
tle with thickened border; anal opening crenulate. 
Type, Unio aratus Lea. 
A peculiar group of small species, which conchologically 
resemble some of the Plagiolas of the Central American region. 
But in the gravid animals the outer gills are filled throughout 
their entire length with embryos. forming a pad-like marsu- 
pium. That of Plagiola is very different, consisting of a limited 
number of ovisacs in the hinder part of the outer gills, which 
are separated from each other by sulci. 
UNIO FALLACIOSUS Nn. sp. 
Shell small, subtriangular, somewhat inflated, rather solid 
with high beaks, whose sculpture consists of a few moderately 
coarse ridges, which are corrugated and somewhat doubly 
looped; from the beaks there is an almost straight oblique 
truncation to the anterior and posterior points; the base line 
is evenly rounded ; posterior ridge strong; surface everywhere 
covered with coarse, rather sharp, concentric sculpture, tawny, 
sometimes greenish-yellow or vellow-green, often having a few 
faint ravs at the beaks; left valve with two rather high, sub- 
compressed, ragged and, sometimes slightly refiected, pseudo- 
cardinals and two laterals; right valve with two pseudocar- 
dinals, the upper feeble, and one lateral; beak cavities shal- 
low, showing a few scars; muscle scars moderately impressed : 
nacre soiled white to salmon. 
Length 32, height 23, diam. 17 mm. 
Nicaragua. 
