8 MEMOIRS OF THE CARNEGIE MUSEUM. 



Rhoads, 1899, p. 13G, has recorded specimens of this species under V. obliqmis from Coraopolis, AUe- 



glienj^ Co., and Beaver, Beaver Co. 

 Ortmann, 19096, p. 200. 



Characters of Shell: Shell large and heavy, swollen, the diameter amounting 

 to fifty percent of the length or more, subcircular or ovate; when old, often oblique 

 and drawn-out at the lower posterior end. No posterior ridge. Beaks moderately 

 i:)rominent. Beak-sculpture not distinctly observed, but probably weakly de- 

 veloped and concentric. Epidermis in young specimens light brown, almost yellow, 

 with more or less distinct dark rays, which often appear as bundles of fine lines, 

 and frequently break up into squarish spots, and with dark concentric growth- 

 lines. In older specimens the epidermis turns darker, brown to blackish, and 

 becomes nearly uniform without any rays. 



Hinge-teeth heavy, pseudocardinals divergent in the young, becoming very 

 heavy, and subparallel to the laterals, in old shells. Interdentum variable, but 

 generally very wide, and beak-cavity very deep and compressed. Nacre alwaj^s 

 whitish. 



L. H. D. Pr.ct. 



Size: Industry, Cat. No. 61.3938a 107 mm. 81 mm. 53 mm. ..50 



This is the largest specimen at hand. It fairly represents the average in 

 outline, but the diameter has been unduly lowered by the production of the 

 posterior end. 



Soft parts and Glochidia (See Ortmann, 1912, p. 244). 



Breeding season: Tachytictic form, breeding in June and July (Ortmann, 

 1909a, p. 101). Gravid females have been found on the following dates: June 22, 

 1909;' June 24, 1909; July 3, 1908; July 11, 1911; July 5, 1909; July 13, 1908. 

 Glochidia as well as eggs were found on July 5 and 13; on the other dates, eggs 

 only.^ The discharge of placentse was observed in a few cases as early as June 24 

 (Ortmann, 19116, p. 306), but this was in captivity, and was certainly premature. 



Remarks: This is one of our heaviest and largest shells. Its external char- 

 acters are rather indifferent, and it resembles several other species, with which 

 it is easily confounded. In general the subcircular or oval outline, with rather 

 evenly curved margins, and the deep, compressed beak-cavities characterize it, 

 Pleurohema ohliquum catillus comes very near to it, however, but this form is 

 mostly more triangular, with the lower margin more nearly approaching a straight 

 line, and with a more distinct lower posterior angle. It has only a rather 

 shallow, and not a compressed, beak-cavity. Of course the characters of the 



^ Of the form from Elk River, West Virginia (var. kucogona Ortmann, 19136, p. 89) I found gravid 

 females as early as May 25, 1911, and as late as .July 10, 1911. 



