716 UNIO 
This form has apparently passed as Unio aratus and some 
time ago I believed it to be the young of Quadrula spheniopsis. 
I am now satisfied that it is neither of these, as it is a solider, 
higher, rather more inflated shell than the former, and has 
coarser, sharper sculpture, while the outline is less quadrate 
than that of spheniopsis at the same age. 
Unio aratus Lea. 
Shell small, subsolid, suboval to subtriangular, somewhat 
convex, with a well-defined posterior ridge, which ends behind 
in a rather sharp point below the median line, anterior end 
slopingly subtruncate above, rounded below ; base rather evenly 
rounded ; epidermis yellowish-tawny, sometimes tinged brown- 
ish or greenish and everywhere strongly and irregularly sul- 
cate; left valve with two somewhat compressed, ragged pseu- 
docardinals and two lamellar laterals; right valve with one 
pseudocardinal, with a vestigial one above it, and one lateral, 
all the pseudocardinals often slightly recurved; beak cavities 
shallow, showing a few posterior scars ; muscle scars not deep ; 
nacre dirty white or yellowish. 
Tength 28, height 18, diam. 13 mm. 
Length 34, height 20, diam 15 mm. 
Central America. 
Unio aratus ea, Disc. 12 Uniones, 1843; Tr. Am. Phil. Soc., 
LX, -1845, p. 282, pls xn, ies T2ag0Obs 7 AVen e148. ps 40. pL 
XL, fig. 12—Sowersy, Conch. Icon., XVI, 1868, pl. Lxiv, 
fig. 320.—SIMPSON, Syn., 1900, p. 744.—VON Marrens, Biol. 
Cent. Amer., Moll., 1900, p. 499, pl. xxxIx, figs. I, Ia, 1D, 2, 
3, 34, 30. 
Margaron (Unic) aratus L&a, Syn., 1852, p. 28; 1870, p. 35. 
Unio nuculinus Puitiprr, Zeit. fiir Mal., V, 1848, p. 176.— 
Simpson, Syn., 1900, p. 607.—von Marvens, Biol. Cent. 
Am., Moll., 1900, p. 509, pl. xxx1x, fig. 6, 6a, 6b. 
This species is a variable one and some of the specimens 
are much more elongated than others and occasional shells are 
fuller at the posterior base than others and these are probably 
females. I have examined the gravid animal of what I believe 
