OETMANN: monograph of the naiades of PENNSYLVANIA. 325 



by their slightly more elevated beaks and their more brownish-olive epidermis, 

 and by specimens of the same character from Black River, as well as by a specimen 

 from the Mississippi at Andulusia, received together with typical orhiculata. 



Genus Truncilla Rafinesque (1820). 

 Ortmann, 1912, p. 354; Simpson, 1914, p. 2. 



Ty\iQ Truncilla triquetra Rafinesque. 

 Simpson (1900) has divided the genus into four subgenera {Truncilla, Scale- 

 naria, Dysnomia, Pilea) while Walker (1910) in a recent revision of the genus, 

 admits three main groups, which properly might be regarded as subgenera. Investi- 

 gations of the soft parts, however, probably wiLl require some alterations of this 

 arrangement. I prefer to ignore the subgenera for the present, since there are 

 only two species in Pennsylvania, the fu-st one (triquetra) being a true Truncilla, 

 while the second (rangiana) would fall under Simpson's Pilea. The rest of the 

 species are chiefly found in the Tennessee-Cumberland region. 



Key to the Pennsylvanian Species of Truncilla. 



Ci. Shell with a very distinct posterior ridge, and distinctly truncate posterior slope. Inflation of female 

 shell narrow, very convex, restricted to the posterior ridge, not differing from the rest of the shell 

 in texture, and denticulated on the margin. Rays strongly spotted T. triquetra. 



02. SheU with a very indistinct posterior ridge, and the posterior slope not truncate. Inflation of the 

 female shell very broad and flat, occupying the whole postbasal area, with the nacreous layer poorly 

 developed, not denticulated on the margin. Rays simple, not spotted T. rangiana. 



Truncilla triquetra Rafinesque (1820). 

 Truncilla triquetra Rafinesque, Simpson, 1914, p. 5. 



Plate XXI, figs. 3, 4. 

 Records from Pennsylvania: 



Rhoads, 1899 (Ohio River, Coraopolis, Allegheny Co., and Beaver, Beaver Co.; Beaver River, Wampum, 



Lawrence Co.) 

 Ortmann, 19096, p. 188. 



Characters of the shell: Small, moderately thick. Outline subovate to sub- 

 trapezoidal and subtriangular. Anterior margin romided. Lower margin gently 

 curved, but sometimes (chiefly in the female) straight or even slightly emarginate 

 posteriorly. Upper margin very short, passing in old specimens into the obliquely 

 descending posterior margin without forming an angle (shape triangular), but in 

 young specimens there is generally an angle (shape trapezoidal) . Posterior margin 

 forming a rather distmct, but more or less roimded, angle with the lower margin. 

 This angle is situated very low, only a httle elevated above the base-line. Beaks 



