270 THE AMERICAN MIDLAND NATURALIST 
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—o.280—(Miss. R., Hannibal, Mo.) 
47 xX 28 x 20—0.2850-(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. ferussacianus 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 Mal., 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 gvisacs undivided; no conglutinates, 
embryos held in mucus masses; glochidium large, spined, sub- 
triangular, hinge line straight, or nearly so. 
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