ORTMANN: monograph of the naiades of PENNSYLVANIA. 65 



by the clam-cliggors out of deep water, with the other bank-forms. This seems 

 to be Its favorite habitat. 



General distribution: Type-locality, Ohio River (Lea) 



This species seems to be restricted to the Ohio, Cumberland, and Tennessee 

 systems, and the known records are all from these rivers and some of tlieir larger 

 nbutanes, as for instance the Chnch in eastern Tennessee, the Wabash in lUinois 

 and Indiana, and possibly also the Illinois River (Fulton Co.) in lUinois (Baker, 

 1906). It has been reported also from the Mississippi in Iowa,- but this is doubted 

 by Simpson. It is to be especiaUy noted, that from Indiana through Ohio and 

 West Virginia to Pennsylvania, this species is restricted to the Ohio proper. 



Plethobasus cyphyus (Rafinesque) (1820). 

 Pkuroherna <.sovus (Green) Simpson, 1914, p. 806; Pleurohema cyphia (Rafin- 

 esque) Vanatta, 1915, p. 556. 



Plate V, fig. 6; Plate VI, figs. 1, 2, 3. 

 Records from Pennsylvania : 



Green, 1827 (rivers near Pittsburgh). 

 Harn, 1891 (western Pennsylvania) 



CMracters of shell: Shell rather large, heavy. Outline elongated-subovate, 

 generally (except m young shells) distinctly longer than high, somewhat oblique, 

 with the beaks not very high, inclined forward, but not at the anterior end of the 

 shell. Beak-sculpture consisting of a few, thick, concentric ridges, not extending 

 upon the disk. Shell moderately swollen, but old shells sometimes more con- 

 siderably so; sides convex, with a broad and shallow radial depression, running 

 from the beaks toward the posterior lower margin, producing a more or less distinct 

 shaUow excavation of the margin. In front of this depression, a radial row of low 

 transverse tubercles running from the beaks to the lower margin. These tubercles 

 begin at a certain distance from the beaks, are more or less distinct (sometimes 

 almost obsolete), and are somewhat irregular, larger or smaller. In rare cases 

 they are cut by obscure, nearly vertical, lines (similar to P. cicatricoms). Behind 

 the radial depression there is a low and broad ridge, passing graduaUy mto the 

 posterior slope. This part of the shell is generally less swollen than the anterior 

 and may be entirely smooth, or may show irregular, low rugosities, and sometunes 



- Call (1885) and Keyes (1888) give Muscatine, Iowa, and Pratt (1876) Davenport low. but it 

 IS not in Witter's (1878) list from Muscatine. ' ' 



