ORTMANN: monograph of the naiades of PENNSYLVANIA. 331 



Truncilla rangiana (Lea) (1838). 

 Truncilla perplexa rangiana (Lea) Simpson, 1914, p. 25.^^^ 



Plate XXI, figs. 5, 6, 7. 

 Records from Pennsylvania : 

 Marshall, 1895 (as U. perplexus Lea. from the Allegheny River, Warren Co.; this undoubtedly is T. 



rangiana). 

 Ortmann, 19096, p. 188. 



Characters of the shell: Shell rather small (but larger than that of T. triquetra), 

 moderately thick, but extremely thin at the postbasal expansion of the female. 

 Outline irregularly subovate. Anterior margin rounded. Lower margin of the 

 male convex in front, straight or concave, and ascending in the posterior part; 

 this posterior part forms a blunt, more or less distinct angle with the anterior part. 

 In the female the lower margin forms a more or less distinctly double curve, the 

 posterior part more strongly convex, and the two parts are separated by a short 

 concavity or notch. Upper margin moderate, passing in a blunt angle or gradually 

 into the obliquely descending posterior margin. In the male the posterior and 

 lower margins meet in a blunt posterior point, which is situated well above the 

 basal line ; in the female the posterior end of the shell is broadly and evenly rounded. 

 Beaks not much swollen, and little elevated above the hinge-line, located in front 

 of the middle of the shell, and only a short distance away from the anterior end. 

 Beak-sculpture rudimentary, consisting of three or four faint bars, which are 

 double-looped. Valves rather compressed, moderately and rather evenly convex 

 in the anterior part of the shell. In the male there is a blunt and indistinct pos- 

 terior ridge, and in front of this a broad and shallow radial furrow, running from 

 the beak to the posterior part of the lower margin. In front of this furrow the 

 shell is often somewhat elevated, forming a blunt rib running toward the angle 

 in the middle of the lower margin. Posterior slope narrow, gently convex or 

 flattened, not truncate. In the female the region of the furrow and of the posterior 

 ridge is occupied by a broad and somewhat flattened expansion, in front of which 

 there is a more or less distinct, narrow constriction in old specimens. Posterior 

 ridge and posterior slope entirely indistinguishable in the female. 



Epidermis yellow or greenish olive, generally with rather distinct, fine, and 

 crowded, straight and uninterrupted, dark green rays. These rays may cover 

 practically the whole surface, but in old specimens they disappear toward the 

 lower margin and upon the posterior expansion, which often is of a lighter color 



=" U. guhernaculum Reeve, 1865, PI. 28, fig. 146, is not this form, as Simpson believes, but a variety 

 of T. tondosa (Rafinesque) (= perplexa Lea) found in the headwaters of the Tennessee River. 



