684 UNIO 
Unio lazarus Lea, Pr. Am. Phil. Soc., V, 1852, p. 251; T'r. Am. 
Phil. Soc., XI, 1852: p. 259). ply saw aie. Os Obs a 1ra52, 
p. 15, pl. xiv, fig. 9 —Srupson, Syn., 1900, p. 732. 
Margaron (Unto) lazarus Lea, Syn., 1852, p. 39; 1870, p. 62. 
A curious form, whose relationships are a little doubtful. 
It may be only a variety of U. arctatus, but it is more arcuate 
than anything I have seen of that species, is not so biangulate 
behind, and has less perfect pseudocardinals. It recalls Mar- 
garitana, but I have found no pits in the nacre of any of the 
shells I have seen. Unio strigosus approaches it in appearance, 
but is much more solid and is less arcuate. 
Group of Unio downtet. 
Shell subtrapezoidal, infiated, solid, truncate and more or 
less triangular behind, sometimes swollen in the post-basal 
region, with a well-developed posterior ridge; beaks full, their 
sculpture not observed; epidermis smooth and shining in the 
young shell, becoming duller and roughened when old; hinge 
moderately strong; pseudocardinals radial, roughened ; laterals 
heavy, somewhat remote, curved; there is a more or less 
developed secondary lateral in the right valve; dorsal scars 
few, in a row just behind the pseudocardinals, and fully ex- 
hibited on the inner edge of the shallow beak cavity; nacre 
dull; muscle scars distinct, smooth. 
Animal with the marsupium occupying the whole of the 
outer gills; inner gills free from the abdominal sac a part of 
their length ; palpi rather small ; mantle line thick on the border. 
UNIO DOWNIEI Lea. 
Shell usually obovate, solid, inflated, inequilateral ; beaks 
full and high, no doubt; posterior ridge well developed, angu- 
lar above, becoming double below and ending at and below the 
median line; anterior end rounded or truncate; dorsal slope 
rather high, scarcely truncated ; base line curved and produced 
behind the middle; epidermis laminated; almost cloth-like, 
dull brownish, sometimes shining a little on the middle of the 
disk; pseudocardinals strong, ragged, two in the left valve 
