ORTMANN: monograph of the naiades of PENNSYLVANIA. 115 



b. Hinge-teeth more or less well-developed, at least the pseudocardinals present. Shell generally 

 not very thin . •' 



b,. Hinge-teeth completely absent. Shell thin, generally very thin. .'.'.".■ ^AnodZa 



a.. Beak-sculpture concentric. Mantle connection between anal a„<l supra-anal openings moderate' 

 .Sometimes a tendency to unite the inner lamina of the inner gills with the abdominal sac. 



6 1. Beak-sculpture fine. Hinge-teeth practically absent inodontoides 



b. Beak-sculpture heavy, often very heavy. Hinge-teeth more or less developed or rudimentary' 

 c. Hinge complete, or at least the pseudocardinals present. Marsupium with simple ovisacs. 



TT- . i, ,. Alasmidonta. 



c. Hinge-teeth rudimentary, but slight traces of pseudocardinals present. Marsupium .-ith 



the ovisacs subdivided into transverse compartments Strophitus. 



Genus Lasmigona Rafinesque (1831). 

 Symphynota Ortmann, 1912, p. 280; Sympkynota, Simpson, 1914, p. 480- Las- 

 migona Frierson, 19146, p. 40. ' 



Type Alasmidonta costata Rafinesque. 

 Simpson has divided this genus into three subgenera, and Ortmann (1914 

 p. 41) has added others, but the latter are extrahmital. The nomenclature of the 

 genus and the subgenera has been revised in this paper in accordance with Frierson 

 (19146, p. 40) and Walker (1918). 



Key to the Subgener.\ of L.\smigoxa. 

 «. Lateral hinge-teeth present. Shell smooth. Beak-sculpture double-looped. Hermaphroditic. 



„ T , , , . ^ ^, , ,. Subgenus Platijnaias. 



a=. Lateral hinge-teeth obliterated. Shell smooth, or with ribs upon the posterior slope. Gonochoristic 



6i. Shell subrhomboidal or subelongate. Posterior slope ^^■ith ribs. Beak-sculpture indistinctly 



double-looped o i r • 



, £,, „ , ,, subgenus Lasmigona. 



b,. Shell broadly ovate-rhomboidal, not elongated, with high posterior ^ving. Posterior slope mth 

 ribs, which may be obliterated. Beak-sculpture sharply double-looped. . . Subgenus Plerosyna. 



Subgenus Platynaias Walker (1918). 

 Symphynota Simpson, 1914, p. 481; Ortmann, 1914, p. 42; Platynaias Walker 

 1918, p. 2. 



Type Unio viridis Rafinesque. 

 Two species belong to this subgenus, which may be distinguished as follows. 



Key to the Species of the Subgenus Platyx.\ias. 

 a I. Shell moderately large, subsolid, compressed. Outline more or less rhomboidal; western habitat. 



ou 11 11 , • ^- (-f-) i-'iridis. 



02. bhell small, tlun, not so much compressed. Outline rather subovate; eastern habitat. 



L. (P.) subviridis. 



