ORTMANN: monograph of the naiades of PENNSYLVANIA. 239 



of the surface. In older specimens the color-markmgs often become obscure, and 

 the epidermis is uniformly brown or blackish. 



Hinge well-developed. Pseudocardinals two in left, one or two in right valve, 

 triangular, elevated and compressed, ragged. Interdentum quite narrow and 

 short. Laterals moderately long. Beak-cavity moderate. Dorsal muscle-scars 

 in beak-cavity. Anterior adductor-scars distinct and well im^iressed, posterior 

 ones faint. Nacre silvery white, rarely pinkish. 



Se.xual differences in the shell verj^ indistmct. Specimens with a more distinct 

 furrow in front of the posterior ridge, and with the posterior section of the lower 

 margin concave, are generally males; while the opposite condition indicates the 

 female sex. In most cases it is very hard to determine the sex from the shell. 



L. H. D. 



Size: 1. Neville Island, Cat. No. 61.1761 58 mm. 43 mm. 27 mm. 



2. Industry, Cat. No. 61.3.569 (cf ) 51 " 40 " 23 " 



3. do. Cat. No. 61.3570 (cf ) 46 " 36 " 22 " 



No. 1 is one of the largest specimens at hand. No. 2 might be a female accord- 

 ing to the shape of the shell, but the soft parts had the male structure. 



Soft parts (See Ortmann, 1912, p. 328). Glochidia: Lefevre & Curtis (1910, 

 p. 97, fig. 1; 1912, p. 146, fig. 3); Surber (1912, PL 2, fig. 30). They are very 

 small: 0.075 X 0.09 mm. 



Breeding season: I found only one gravid female, with glochidia, on May 

 25, 1914. Lefevre & Curtis (1912, p. 141) list this species with the bradytictic 

 forms. Surber (1912, p. 6) states that gravid females with glochidia were found 

 in May and Julj'. 



Remarks: The species is easily recognized by its subtriangular shape, its sharp 

 posterior ridge, truncate posterior slope, and the peculiar color-pattern of the 

 epidermis. It only resembles the following species, but is distinguished from that 

 bj^ the higher and shorter shell, which also gi'ows to a larger size. In addition this 

 species has a certain resemblance to the male of Truncilla triquetra both in general 

 shape as weU aS in color. However Truncilla triquetra is a more elongated shell, 

 the posterior slope is broader, and more distinctly truncate, and the shell is more 

 inflated. 



The posterior ridge of A. truncata is quite variable. In Pennsjdvania it is 

 not so sharp as elsewhere. From Louisiana, for instance, I have specimens in which 

 it is keel-like. The form from Lake Erie is smaller and lighter, but verj' variable 

 in color. Ha\'ing not been found on the Pennsylvanian shores of the lake, I shall 

 not here discuss tliis form. 



