Occasional Papers of the Museum of Zoology af 
much greater than the divergence of Q. nickliniana from some 
of its forms. 
Elliptio (Leptonatas) ravistellus (Morelet) (1849).—One 
dead specimen along Quebrada Laja (H, vii, a). Measure- 
ments: length, 48.5 mm.; height-index, 51 per cent (25 mm,) ; 
diameter-index, 33 per cent (16 mm.). 
The specimen somewhat resembles this species, although a 
larger set might show constant differences. In this individ- 
ual, the general shape approaches that of E. popeti (Lea); it 
is somewhat compressed laterally and has a tendency to be 
subsinuate ventrad. The ridges are closer and finer than in 
typical Z. ravistella, so that the dry shell has a grayish appear- 
ance. When wet, the golden-yellow ground-color, with rather 
diffuse, olive-green rays, is apparent. The posterior, dorsal 
region is marked by two quite deep furrows, and between 
these the surface is wrinkled irregularly, in a manner remotely 
suggestive of the Sphenonaias group. The pseudocardinals 
are lamellar and very oblique, especially in the right valve. 
Unio ravistellus Morelet is the type of the section Leptonaias 
Fischer and Crosse (1894), which I use here in the sense of 
Nephronaias Frierson (1917). Unfortunately, Simpson (1900) 
chose Nephronaias Crosse and Fischer (1894) (type U. plica- 
tulus Charp. in Kuster, 1856) as the name of his mixed 
assemblage of Elliptio-like and Lampsiline species. Frierson 
separates these and retains the name for the unionine shells, 
on the supposition that U. plicatulus is such a species. Von 
Martens (1900), however, places this species in the synonomy 
of A. aztecorum (Philippi). Personally, I think it is rather 
A. medellina (Lea). It is true that the surface looks like the 
Elliptio-group of Southern Mexico and Central America, but 
all of Kuster’s figures look remarkably alike; also the descrip- 
tion mentions fine, close-set wrinkles, but these are also present 
