THE NAIADES OF MISSOURI II9 



times rayed; hinge teeth rudimentray, usually lacking; muscle 

 scars faint, confluent; nacre whitish or pearl blue. 



Length Width Diameter Um. ra. Locality 



48 X 29 X 18 — 0.280 — (Miss. R., Hannibal, Mo.) 

 47 X 28 X 20 — o.285o-(Miss. R., Hannibal, Mo.) 



Miscellaneous Remarks: — ^Probably this species can be 

 best characterized by its medium sized, sub-elliptical shell with 

 its concentric, umbonal sculpturing, polished olive-green or 

 brownish epidermis and slightly incurved hinge in front of beaks. 

 A. ferussacianits is only known to occur for a certainty in the 

 Mississippi of this state. Old, eroded shells from the Niangua 

 River, Camden, Co., and from Casteel Creek, Clinton Co., have 

 structures of this species more than anything else, but the identi- 

 fication is too doubtful for much consideration. The shell of 

 this species is like that of Strophitus, but differs in beak sculpture; 

 it also differs from some similar shells of Anodonta by the incurved 

 anterior hinge to the beaks and by a compressed post-dorsal 

 portion of shell back of a slight posterior ridge. Ortmann (1921b, 

 pp. 293-294) denies that all four of its gills are marsupial, and 

 Simpson, who claims to have found embryos in all four gills does 

 not class this species under his Tetragenae (i. e., those that have 

 all four gills marsupial) because its characters of shell and nutri- 

 tive soft parts seem to agree better with his Homogenae {Unios 

 with only outer gills marsupial.) This species is generally distri- 

 buted throughout the Mississippi drainage, the St. Lawrence 

 system and that of the Red River of the North. The subspecies 

 only occur in the St. Lawrence drainage. 



Genus Alasmidonta Say. 

 1818 — Alasmidonta Say, Jl. Ac. N. Sci. Phila., I., p. 459. 

 1840 — Uniopsis Swainson, Tr. on Mai., p. 382. 



ANIMAL CHARACTERS. 



Mantle connection between anal and supra-anal openings 

 moderately long; inner laminae of inner gills free from visceral 

 mass, or, more or less connected to it; outer gills only marsupial; 

 when charged, distended at ventral edges, water canals facing 

 laminae present, central ovisacs undivided; no conglutinates, 

 embryos held in mucus masses; glochidium large, spined, sub- 

 triangular, hinge line straight, or nearly so. 



