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NAIADES OF MISSOURI 129 
with late embryos and immature glochidia in midwinter, and 
with mature larvae in late winter but barren from April to August. 
Ovulation has been observed for the latter part of August and 
unfertilized ova have been found in the ovaries August roth. 
Sperm having been found within the visceral mass without being 
accompanied by ova for those individuals that possess gills without 
the crowded septa of the female proves the sexes distinct and 
separate and thus disproves the claim of hermaphroditism for 
this species. It is true that during the height of the breeding 
season that all individuals found seem to be females but this is 
the time when there is greater activity among females; hence, 
they may be more in evidence while the males remain inactive 
and burrowed from ready accessibility during this season. This 
fact may account for the so-called hermaphroditism among other 
species. By laboratory tests the writer has kept the glochidia of 
this species alive in cold, clear, fresh water exactly thirty days 
(five time longer than the life of any other mature glochidia sub- 
mitted to this watch-glass test) after being taken from the mother. 
This unusual vitality of the larvae is an adaptation to its pro- 
longation of breeding season into late winter when they are dis- 
charged into the ice-cold water and left to their fate, for it is the 
belief of the writer that they are discharged as soon as mature. 
Genus Quadrula Rafinesque. 
1820—Quadrula Rafinesque, Ann. Gen. Sci. Phys. Brux., p. 305. 
1852—Rotundaria Agassiz, Arch. fur, Naturg., p. 48. 
(Type, Obliquaria (Quadrula) m2tanevra (Rafinesque). 
ANIMAL CHARACTERS:—Branchial opening large with short 
arboreal papillae; anal smooth to. finely dentate; supra-anal 
very large, briefly and loosely connected to anal by mantle edges; 
inner laminae of inner gills free from visceral mass; palpi large, 
somewhat sickle-shape; color of soft parts not bright, except for 
brownish gills and palpi, tannish or soiled white; marsupia occu- 
pying all four gills, when gravid ovisacs swell moderately in center, 
ventral edge obtusely pointed; conglutinates white, leaf-like, 
sometimes divided at distal ends; glochidia small to medium 
in size, subovate, spineless. 
SHELL CHARACTERS:—Shell roundly quadrate, or subrhom- 
boidal, occasionally elongate with moderately high beaks sculp- 
tured with 3-4 parallel ridges developed on post ridge to nodules; 
