ORTMANN: monograph of the naiades of PENNSYLVANIA. 61 



Rivers, and in Ohio by Stcrki (1907a) from the Ohio, Little Miami, and Tuscarawas 

 Rivers. In addition, it is found in southern Michigan (Walker, 1898), and in 

 Lake Erie (Barnes, 1823; Walker, 1913). Here we have apparently the only 

 region, where this species leaves the Mississippi-drainage, and it is quite clear, that 

 this was done by the Wabash-Maumee route. On the Pennsylvanian shores of 

 Lake Erie it has not as yet been found. 



Genus Plethobasus Simpson (1900). 

 Ortmann, 1912, p. 259; Simpson, 1914, p. 805 (as section of Pleurobema). 



Type OUiquaria ajphya Rafinesque. 

 Three species are known to belong to this genus, of which only two are found 

 in Pennsylvania. But since the third turns up in the Ohio, not far from the western 

 state-line, I give here a key for all three of them. 



Key to the species of Plethobasus. 



a I. Shell rounded, ovate, or oblique, without .i radial depression running toward the posterior basal 

 margin. 

 hi. Shell subrotund, or subovate, only slightly oblique. Disk with scattered, irregularly disposed, 

 rounded, or slightly transverse nodules, leaving free the anterior part of the shell, but generally 



extending more or less upon the posterior slope p. cooperianus. 



62. Shell subovate, very strongly oblique. Disk with more or less transverse tubercles, or sub- 

 concentric, more or less interrupted ridges, restricted to the middle of the shell, leaving the 



anterior part as well as the posterior slope free of sculpture P. cicalricosus (e.\trahmital). 



a.. Shell obliquely elongated, with a broad and shallow radial depression running toward the post- 

 basal margin. In front of the depression a radiating row of low and broad, often transverse, 

 tubercles; sometimes also a few indistinct tubercles behind the depression P. cyphyus. 



Remarks as to nomenclature and synonymy: Frierson (1911) has published a 

 paper dealing with these forms, but in my opinion he starts from an incorrect 

 assumption. This assumption is, that IJ. cicatricosus Say (1829) is the same as 

 U. a^sopus Green (1827) (which in turn is identical with cyphyus Rafinesque, 

 1820). The original description of Say mentions two important characters: 

 tubercles and anterior position of beaks (great obliquity), which do not leave the 

 sHghtest doubt, that the species was correctly understood by subsequent writers 

 {U. cicatricosus Reeve, 1864, PI. 8, fig. 31, and Simpson, 1900). 



Reeve figures (C/. PI. 13, fig. 50) a smaller and less elongated individual. 

 Frierson believes that this is another species. If that were the case, I would be 

 compelled to make three or four additional species out of my (scanty) material of 

 cicatricosus. 



There appears to me to be no ground for Frierson's introduction of the new 



