NAIDAES OF MISSOURI 123 
Central Missouri, where ecological conditions are the most diverse 
for the State, the author has been able to observe the same occur- 
rence of the Amblemae as noted by Wilson and Clark in the Cum- 
berland River (1914, p. 21) in that the more plicated and less 
inflated (undulata) one will be found in upper courses, while the. 
smoother and more inflated (plicata) one is confined to the lower 
portions of the.rivers where there is more mud and a weaker 
current. On the basis of not only these state-wide observations 
but also on these as limited to a single river, we would account 
for the existence of these two opposing types of Amblema as due 
to ecological rather than to genetic causes. However, as juvenile 
shells of two forms are different their origin would also indicate 
difference and the matter of their occurrence under certain ecolo- 
gical relations might, after all, be simply one of survival. A careful 
study of Amb. costata shows it to be a summer breeder, beginning 
in May and closing the latter part of July. As this ‘“‘undulata”’ 
group has been understood better taxonomically than the “ plicata”’ 
the geographic distribution of costata has also been better deter- 
mined. Simpson reports it (i. e., his QO. undulata (Barnes)) for the 
Mississippi basin generally; also for the drainage basins of the 
St. Lawrence, the Red River of the North and the Alabama River. 
The varieties of this species, however, are reported by many for 
the area south and west of the Mississippi River known as the 
“South-West,” the fauna of which is included in Central and 
South Missouri and bounded on the north by the great faunal 
barrier, the Missouri River. 
Genus Megalonaias Utterback. 
(New Genus.) 
Type, Unio heros Say, 1829. 
ANIMAI, CHARACTERS. 
NUTRITIVE STRUCTURES:—Branchial opening very large with 
short papillae; anal and supra-anal also large, almost smooth, 
separated by short but distinct mantle connection; inner laminae 
of inner gills partly free from visceral mass or sometimes almost 
entirely connected; palpi long, enormous; color of soft parts, 
tan colored, with gills brownish. 
REPRODUCTIVE STRUCTURES:—Marsupia occupying all four 
gills, when gravid enormous, padlike, not so distended at ventral 
