28 Trans. Acad. Sci. of St. Louis. 



west, is not only abundant, but is frequently the only form 

 found. It is usually, also, of a darker color, and inhabits 

 muddy portions of the river beds. The female shells are 

 usually found in greater abundance than the male forms, but 

 occasionally the latter are most numerous. The female 

 appears to have formed the basis of nearly every described 

 species that has been erected, and which appears in the syn- 

 onymy given herewith. 



Unio metanevrus Eafinesque. 



Plate X. 



Obliquaria metanevra Rafinesque, Annales Generales 

 des Sciences Physique, Bruxelles, September, 1820, p. 

 305, PI. LXXXI, Figs. 15 et 16. Described from 

 Kentucky. 



Uhio metanevra Rafinesque, so Reeve, in Conchologia 

 Iconica, Unio Plate VII, Fig. 25, 1864. A good figure 

 of outside character. 



Uhio nodosus Barnes, Am. Jour. Sci. and Arts, Ist 

 series. Vol. VI, 1823, PI. VI, Figs. 7a and 7b, p. 124. 

 Described from Wisconsin ; Hildreth in Am. Jour. Sci. 

 and Arts, 1st series. Vol. XIV, p. 281, Fig. 10, 1828. 



Unio rugosus Barnes, Am. Jour. Sci. and Arts, 1st 



series, Vol. VI, 1823, p. 126, Fig. 9. Described from 



Ohio; Hildreth in Am. Jour. Sci. and Arts, Vol. XIV, 



1st series, p. 282, Fig. 12, 1828. 



Our Arkansas specimens came from the St. Francis, at 



Wittsburg ; the Ouachita, at Malvern ; the Saline, at 



Benton. 



A small number of related forms, constituting a natural 

 group, are headed by this earliest described member ; among 

 them are Unio tuherosus Lea; Unio wardii Lea, and Unio 

 cylindricus Say. The type of the group has a wide distribu- 

 tion from Ohio, south to the Coosa and Alabama rivers, 

 Alabama; Louisiana and Texas; west to the Neosho and 

 Elk rivers, Kansas; and north to Dresbach, Minnesota. 



This species is exceedingly variable. In the Alabama, 

 Tennessee, Cumberland, and Mississippi rivers it is usually 



