NAIDAES OF MISSOURI 4I 



Central Missouri, where ecological conditions are the most diverse 

 for the State, the authcf 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 Q. 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. 



ANIMAL 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 



