THE NAIADES OF MISSOURI 271 
SHELL CHARACTERS. 
Shell subquadrate to subtrapezoidal, thin, inflated; disk 
smooth; beaks heavily sculptured with irregular concentric bars— 
the later ones being more or less undulate; epidermis olivaceous to 
burnt orange with broken rays; cardinals present, laterals reduced; 
beak and branchial cavities deep; nacre white to pearl blue. 
The characters of the shell of this genus—especially in its 
coarse concentric beak-sculpture—shows that it is somewhat 
primitive, yet the tendency of the union of the inner laminae of 
the inner gills with the visceral mass in an indication of progress 
in structure. The two species that represent Alasmidonta for this 
State are not found in the interior north of the Missouri River. 
Alasmidonta calceolus (Lea). 
(“Slipper Shell.’’) 
Pl. XXIV, Figs. 79 A—D. 
1830— Unto calceola Lea, Tr. Am. Phil. Soc., III, p. 265, pl. III. Fig. 1. 
1898—Alasmodonta deltoidea Baker, Moll. Chicago, Pb. I., p. 63, 
Bi Wil, ime 2c VL ae, A: 
1900b—Alasmidonta calceola (1rea) Simpson, U. S. Proc. Nat. Mus., 
XXII, p. 668. 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTURES:—Siphonal openings large, mantle 
edges with regular blocks of black; gills of medium size, inner 
laminae partly united with visceral mass; palpi rather long, 
tongue shaped; most of soft parts light yellowish. 
REPRODUCTIVE STRUCTURES:—Marsupium  pad-like and 
brownish when charged, water-canal present; glochidium large, 
spined, spadiform, longer than high, hinge line straight. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell small, slipper-shaped or sub- 
trapezoidal, obtusely angular behind, post-umboidal ridge rounded; 
disk smooth; beaks high, pointed, with coarse concentric sculp- 
turing consisting of four or five bars sharply bent in behind; 
epidermis yellowish or olivaceous with wavy double rays on and 
parallel to the post umbonal ridge. 
INTERNAL STRUCTURES:—Cardinals single and conical in 
right valve, double in left with post tooth saddle-shaped; laterals 
mere thickened hinge line; nacre white. 
