338 
The American Naturalist. 
[April, 
LEA’S DESCRIPTION OF Unio pressus AND Margaritana rugosa. 
Unio pressus Lea. 
Branchial uterus occupies the 
whole of the outer branchie. 
Branchiz large, rounded be- 
low, free nearly the whole 
length of the abdominal sac. 
Palpi small, subangular, 
united half way down the pos- 
terior edges. 
Mantle thin, slightly thick- 
ened on the margins. 
Branchial opening large, 
blackish on the edge, and with 
numerous papille. 
Anal opening rather small, 
blackish, and without pa- 
pillæ*. 
Superanal opening rather 
large, united for some distance 
below, blackish on the edges. 
Color of the mass dirty 
white. 
Embryonic shell subtrian- 
gular, light brown, has hooks. 
Margaritana rugosa Bar. 
Branchial uterus occupies the 
whole of the outer branchie; 
brownish, forming a large, 
massive lobe which extends 
below the margin. 
Branchie very large, round- 
ed below, the inner ones much 
the larger, free nearly the 
whole length of the abdominal 
sack. 
Palpi rather small, subtri- 
angular, united nearly one- 
half way down the posterior 
es. 
Mantle rather thin, much 
thicker at the margin, black- 
ish on posterier basal edge. 
Branchial opening rather 
large, with small, brown pa- 
pillee. 
Anal opening rather large, 
without papille. 
Superanal opening very 
large, with a dark brown line 
within, united below. 
Color of the mass salmon. 
Embryonic shell triangular, 
brown, has hooks. 
* Agassiz claimed that two distinguishing characters of the genus Margaritana 
were (Archiv. fur Naturgeschichte, 1852, I, p. 41) that certain species 
had gills 
free from the mantle at their posterior extremities, and that the anal region was 
not fringed, while in the typical Unios the branchie and mantle were united pos- 
teriorly, and that both siphonal openings had papillae. According to this, Unio 
pressus and its allies, though having lateral teeth, would be Margaritanas. 
