74 ^rEMOIRS of the CARXEGIE MUSEUil. 



The existence of typical P. obliquum becomes uncertain in the west and south- 

 west. It has not been recorded by Scammon (1906) from Kansas, and although 

 Call (1895, p. 32) cites it from the Ouachita River in Arkansas, I have never seen a 

 genuine P. obliquum in all the rich material I have received from H. E. Wheeler 

 taken from this river. In this region it seems to be entirelj' replaced by the vars. 

 catillus and rubrum. Utterback (1916) failed to find this species in Missouri. 



From what we know at present it is quite e\'ident that the species prefers the 

 larger rivers (See Call, 1900, p. 502) and thus it is easily vmderstood why it has not 

 migrated from the Mississippi-Ohio system into any other drainage. 



Pleurobema OBLiQurii CORDATUM (RafiLnesque) (1820). 

 Quadrula plena (Lea) Simpson, 191-1, p. 886; Quadrula cordata (Rafinesque) 

 Vanatta, 1915, p. 558. 



Plate Yll, fig. 1. 



No records from Pennsylvania pre\'ious to those given below. 



Characters of variety: ^luch hke t}"pical obliquum, but shape more rounded, 

 which is due to the sUght development of the lower posterior angle and the whole 

 posterior slope. In consequence of this, the shell appears more elevated and more 

 upright. With regard to the diameter, the shell resembles P. obliquum (over fiftj* 

 percent of length). The radial furrow is generally weak, but present. Xacre 

 white, or sUghtly pink. 



L. H. D. Pr.ct. 



Size: 1. Cooks Ferry, Cat. No. 61.443-1& 78 mm. 79 mm. 51.5 mm. .66 



2. Godfrey, Cat. No. 61.39016 70 " 67 " 43 " .61 



3. Godfrey, Cat. No. 61.4378 56 " 57 " 34 " .61 



Soft parts and glochidia not observed in Pennsylvania. In the region of the 

 upper Tennessee I found a few gra\id females, but none with glochidia. The 

 anatomy is the same as in the main species. Only the outer gills possess marsupial 

 structure. 



Breeding season: Gra\-id females were found on May 25, 1914. 



Remarks: This form hardly deserves a varietal name, and I retain it only 

 because it has been distinguished as a "species" by pre\'ious authors. In Penn- 

 sj'lvania, it is nothing but an individual variation, found verj- rarely, and always 

 associated and intergrading with the typical form. The three specimens, of which 

 measurements are given, are the only ones I have from Pennsylvania, and even 

 these are not quite topical representatives of cordatum. In the upper Tennessee 

 region, I have found this form more frequently, but also there it is by no means a 

 weU-defined race, and I know of no region, where cordatum is found pure. 



