284 Annals of the Carnegie Museum. 



doubtedly a Symphynota, but the beak-sculpture is abnormally de- 

 veloped. The sinuation of the bars, however, indicates that this 

 species originally had double-looped sculpture. 



Color of soft parts rather remarkable. Orange tints are often found, 

 similar to those seen in certain forms of Alasmidonta. The ground- 

 color is yellowish brown; foot, margins of mantle, and adductors, 

 often deep orange. The gills are brown, the edge of the mantle, as 

 usual, blackish. The marsupium, when charged, varies from yellowish 

 to brown. 



Genus Arcidens Simpson. (1900.) 

 Simpson, 1900&, p. 661. 



Shell subrhomboid, inflated, with full beaks. Disk sculptured. 

 Beak-sculpture strong, distinctly double-looped, the loops tubercular, 

 and the tubercles are continued in two radiating rows upon the disk. 

 In addition, there are oblique folds upon the disk, and the posterior 

 slope is also sculptured. Hinge with teeth, pseudocardinals present 

 and well developed, laterals obliterated, but traces of them may be 

 seen. 



Soft parts, as far as known, anodontine in structure, and similar to 

 the genus Symphynota. Gravid females have not been observed. 



Type: A. confragosus (Say). 



The genus is incompletely known, but very probably it is to be placed 

 near Symphynota. 



Arcidens confragosus (Say). 



I have one male and two females from Bayou Pierre, De Soto Parish, 

 Louisiana, collected on Aug. 6, 1910, by L. S. Frierson, further the 

 gills of an additional female from the same place, collected a little 

 later, and one female from Pearl River, Jackson, Hinds Co., Mississippi, 

 collected Nov. 5, 1910, by A. A. Hinkley. None of the females was 

 gravid. 



The soft parts of the male have been described by Lea (Obs., N, 

 1863, p. 448). Supra-anal long, well separated from the anal by a 

 mantle-connection, which is shorter than the anal. Inner edge of 

 anal crenulated, inner edge of branchial with papilhe. Diaphragm 

 complete and normal. Inner lamina of inner gills free, except at 



