t2o The naiades of Missouri 



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.") 



PI. XXIV, Figs. 79 A—D. 



1830 — Unio calceola Lea, Tr. Am. PhiL Soc, III, p. 265, pL IIL Fig. i 

 1898 — Alasmodonta deltoidea Baker, MolL Chicago, Pb. I., p. 63, 



PL VI, fig. 2; VII, fig. 4- 

 1900b — Alasmidonta calceola (Lea) Simpson, LT. 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. 



