ORTMANN: monograph of the naiades of PENNSYLVANIA. 221 



02. Shell not higher than long. Beaks less elevated and not much incruved. Nacre wlute or purplish. 



61. Shell swollen. Diameter sixty percent of the length or more 0. subrolunda. 



62. Sliell compressed. Diameter less than si.xty percent of length 0. subrotunda levigala. 



Obovaria (Obovaria) retusa (Tjamarck) (1819). 

 Obovaria retusa (Lamarck) Simpson, 1914, p. 290. 



Plate XIII, figs. 6, 7. 

 Records from Pennsylvania: 



Ortmann, 19096, p. 192. 



Characters of shell: Shell of medium size, heavy and solid. Outline subcircular 

 or subovate, about as high as long, or even higher than long, upright. Anterior, 

 lower, and i^osterior margins almost regularly roimded, only with a blunt lower 

 posterior angle in the male, and a slight emargination in this region in the female. 

 Upper margin short, convex. Beaks swollen and elevated, situated near the middle 

 of the shell, but strongly incurved forward. Beak-sculpture rudimentary, never 

 distinctly seen by the writer. Valves strongly convex, without a distinct posterior 

 ridge. No sculpture on the outer surface, but irregular concentric ridges, corre- 

 sponding to the growth-lines, are often present. 



Epidermis light to dark brown, generally quite uniformly dark brown upon 

 the disk, while the posterior slope is lighter, pale brown or yellowish. The lighter 

 color is not sharply marked off from the darker part, and often the contrast is 

 not evident at all. Upon the lighter posterior slope, there are sometimes a few 

 feeble greenish rays, and in young specimens, such rays may be indicated by mere 

 traces upon the disk, but normally there are no rays at all. 



Hinge well-developed. Pseudocardinals very heavy, stumpy, ragged, tri- 

 angular, one in right, two in left valve, the latter divergent and not parallel to 

 the laterals. Interdentum broad and short. Laterals short, thick and heavy'. 

 Beak-cavity rather deep and narrow. Dorsal muscle-scars upon the inside of the 

 hinge-plate. Adductor-scars distinct and deeply impressed. Nacre of various 

 shades of purple, generally very deeply colored, but the margin outside of the 

 mantle-scar is generally white. 



Sexual differences of the shell moderate, but distinctly marked. In the 

 male, the lower and posterior margins form a regular curve or meet in an indistinct, 

 rounded lower posterior angle. In the female the disk shows a slight swelling, 

 forming a slight projection in the ])osterior part of the lower margin situated at a 

 lower level than the posterior angle of the male. Above this swelling, and behind 

 it, the shell is slightly depressed, and the posterior margin ascends straight or is 



