86 
Psyche 
[September 
chambers of the clams, not inserted into the mantle or gills as in 
Unionicola. The greatest number of egg masses found in a single 
clam was 23 in one A. cataracta. Each egg mass contained approx- 
imately 250-300 eggs, and was enclosed in gelatinous material, lying 
unattached in the suprabranchial chamber. The color of the eggs 
varied from light cream when freshly laid, through orange, to nearly 
black when about to hatch. Masses of light cream-colored eggs were 
hatched in 17 days in distilled water at summer room temperature. 
Sexual dimorphism is very pronounced in N. ingens , gravid 
females reaching a diameter of 5 x 6 mm., 3 or 4 times that of males. 
The color varies from honey yellow to light reddish brown. Micro- 
scopically the sexes may be recognized by the position of the pair of 
triangular genital plates. Those of the male are contiguous medially, 
while in the female the two plates are separated. The average num- 
ber of acetabula per plate in 10 males was 72 (39-142) and in 10 
females 94 (59-122). The two acetabula located near the outer apex 
of the plate are larger than the others. 
Adult N. ingens can survive for several weeks at least outside the 
clam host. In distilled water at summer room temperature a male 
survived 36 days and a female 49 days. The female laid eggs during 
this period. 
Fig. 4. Anodonta cataracta with left valve and mantle removed and 
left outer suprabranchial chamber opened to show papillae, a male and 
a female N. ingens, and several small, dark egg masses, (xl.5) 
