160 MEMOIRS OP THE CARNEGIE MUSEUM. 



earlier, and missing in the later bars. This holds good in at least one of the speci- 

 mens before me. The rough epidermis is also given as a character, as distinguished 

 from the shining catarada, but this is not confirmed by my material. 



L. H. D. 



Size: 1. Yardley, Cat. No. 61.3639 75 mm. 41 mm. 26 mm. 



2. do. " " do 74 " 40.5 " 25 " 



These are the only specimens of considerable size collected by myself, and 

 they are hardly more than half the maximum size attained by this form. 



Soft parts and glochidia unknown. 



Breeding-season: According to Conner (1909) from September to May (like 

 cataracta). Lefevre & Curtis (1912, p. 141) enumerate this species under the forms 

 with long breeding-season. 



Remarks: This is a form the taxonomic standing of which is as yet obscure 

 and I myself have not been able to form a clear conception of it. According to 

 previous authors, the characters given above are essential, but they apply only 

 to the adult shell, and I am unable to say much about the young shell. The 

 smallest shells I have seen are those I collected in the Delaware at Shawnee (Length : 

 58, 46, 41, and 39 mm.). Of these the largest shows traces of the thickening at 

 the lower anterior margin, while the smaller ones do not. These sheUs have the 

 epidermis rather yellowish, and the nacre has a good deal of salmon-color, and 

 therefore I think they actually represent this species. All other shells I have 

 seen are larger, and show the character of the thickening of the margins well- 

 developed. The best account of this shell has been given by Gould (Binney, 

 1870, p. 180), but even here it is admitted that the young shells are difficult to 

 distinguish from A. fluviatilis (= cataracta). 



The Carnegie Museum possesses only eight specimens, to which I should apply 

 the name implicata. One (without locality) is from the Hartman collection, and 

 is certainly this form. It is large (L. 122 mm.), elongate (H. 67), and swoUen 

 (D. 56). Its epidermis is shining (not rough) and of a yellowish-olive to light 

 brown color, without any rays. The beak-sculpture is as described by Marshall. 

 The valves also show the characteristic thickening along the anterior lower margin, 

 and the nacre is pale pinkish. Another specimen was sent to me by C. H. Conner 

 (Timber Creek, Gloucester, New Jersey, just south of Newton Creek, Camden 

 Co., the type locality of Lea's newtonensis) . It is 103 mm. long, 49 high, and 41 

 in diameter. Thus it is rather elongated, swollen, and subcylindrical. The 

 epidermis is blackish brown (not pure black as in the figure of newtonensis), the 

 beaks are eroded, and the beak-sculpture gone. The thickening of the margin 

 is present, and the nacre is pinkish or sahnon. 



