IlSo TRAl'KZOIDF.US 



Genus TRAPEZOIDEUS Simpson. 1900. 



Trapccoideits Simpson, Syn., 1900, p. 858. 



Shell trapezoid, much compressed, with a low posterior ridge, 

 and hut sliyhth raised, though pointed, beaks, which seem to be 

 sculptured with irregidarly radial ridges arranged in two im- 

 perfect chevron-shaped loops; surface concentrically sculp- 

 tured, often having irregular radial ridges on the posterior 

 slope; epidermis yellowish-green or brownish, with two or 

 more green rays above the posterior ridge ; teeth compressed ; 

 two pseudocardinals in the left valve, one under the beak, the 

 other in front, often not well separated, and two laterals ; 

 right valve with two pseudocardinals and one lateral; pseudo- 

 cardinals all irregular, often pitted and peculiarly dentilate ; 

 laterals granular, showing traces of vertical striation; cavity 

 of the beaks rather deep, compressed; muscle scars shallow, 

 anterior scars separate ; nacre soft, creamy, yellowish in the 

 beak cavities. 



Animal unknown. 



Type, Unio foliacea Gould. 



The species which I have placed in this group have generally 

 elongated, rhomboid or trapezoid, rather thin, subcompressed 

 shells. The beaks are but slightly elevated and nothing is 

 known of their sculpture so far as I know. The surface has 

 well-marked, concentric sculpture, which is sometimes cor- 

 rugated, the teeth are compressed and roughened, the pseudo- 

 cardinals being usually rather feeble. The group does not 

 seem to be very close to any other. 



Kiev To SPECIKS OF Trapezouieus. 



Shell with dark cjiidermis and nacre. 



Long rhomboid. T. cxolescens. 



Ovate elliptical. T. ludovicianus. 

 Shell with light epidermis and nacre. 



Trapezoid. T. pallcgoixi. 

 Obovate rhomboid. T. theca; peninsiilaris. 

 Rhomboid. 



Not arcuate, T. foUaccus. 



Subarcuate. T. miscellus. 



