NAIADES OF MISSOURI • 47 



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 19th. 

 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) metanevra (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; 



