NO. 2122. THREE NEW BRAZILIAN ANODONTITES— MARSHALL. 529 



ANODONTITES AURORA, new species. 

 Plate 69. 



Shell elongately subquadrate, regularly rounded in front, angularly 

 rounded, but somewhat attenuated, posteriorly. Early shell with 

 concentric, channeled lines, posterior dorsal area with evenly spaced, 

 concentric, channeled lines which fade out just in front of the posterior 

 ridge. Anteriorly the shell is nearly smooth. Posteriorly there is 

 a succession of low, broad concentric ribs. Color brownish-oUve, 

 glossy, darkening anteriorly to light brown; posterior area very 

 dark brown, almost blackish, the dark color here sharply differen- 

 tiated from the lighter color of the rest of the shell; entire surface 

 more or less marked with radiating brown lines varying in width and 

 intensity; rest periods five, indicated by darker brownish lines. 



Nacre highly iridescent, with a lurid cast, marked by innumerable 

 radiatmg straiae, which are in the texture of the shell. Cavity of the 

 beak shallow. Upper portion of nacre, including the hinge line, 

 blotched witli lavender. Prismatic margin very narrow, bluish-white. 

 Hinge line nearly straight, making an abrupt angle with the anterior 

 margin of the shell, but scarcely any angle with the posterior margin. 



The type. Cat. No. 273689, U.S.N.M., consists of a single valve 

 which measures: Length, 110 mm,; height, 86 mm.; diameter, if 

 both valves were present, would be 34 mm. It and one other valve 

 come from Ceara, Brazil. 



Superficially this species seems to be most nearly related to Ano- 

 dontites trapezialis Lamarck, but a careful study of form, nacre, 

 color, and other characters shows a closer relationship to Anodontites 

 sinuosus Lamarck. Anodontites trapezialis narrows in front to such 

 a degree as to give the shell a generally oblique appearance. The 

 narrowing in A. aurora is much less, and "hence the shell has a more 

 quadrate form. The difference in the thickness of the two species 

 is very great, trapezialis being much the thicker and much heavier. 

 The nacre of trapezialis is dull, bluish-white while that of aurora is 

 highly iridescent and has a lurid cast. In nacre A. aurora agrees 

 in practically all respects with A. sinuosus. 



A. sinuosus has a generally pathologic appearance. The nacre is 

 blotched with purplish-brown and the posterior portion of the shell 

 is rudely constructed and looks like a "bad job." A. aurora has the 

 purplish blotches on the nacre, but the posterior portion of the shell 

 neatly finished. It is possible that A. sinuosus if grown to perfec- 

 tion would possess the elongated form of A. aurora and that the 

 latter would then prove to be a variety of the former. 



Tlie name aurora is appropriate for the species because of the play 

 of brilliant colors in the nacre. 



81022°— Proc.N.M.vol.49— 15 34 



