Occasional Papers of the Museum of Zoology 7 



much greater than the divergence of Q. nickliniana from some 

 of its forms. 



Blliptio (Leptonaias) ravistellus (Morelet) (1849). — O^^ 

 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. popeii (Lea) ; it 

 is somewhat compressed laterally and has a tendency to be 

 subsinuate ventrad. The ridges are closer and finer than in 

 typical B- 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- 

 tulns Charp. in Kiister, 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. aztecoriim (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 Kiister's figures look remarkably alike ; also the descrip- 

 tion mentions fine, close-set wrinkles, but these are also present 



