PRISODON I 2 19 



Unio syniiatppJiora Dkshayes, Tr. Elem., II, 1853, p. 219, pi. 



XXIX, figs. 10, II. 

 Margarita (Unto) synnatophorus Lea, Syn., 1836, p. 11 ; 1838, 



P- 13- 

 Prisodon syrmatophorus Simpson^ Syn., 1900, p. 870. 

 f Paxyodon pondcrosus Schumacher, Ess. Nouv. Syst., 1817, 



p. 140, pi. XI, fig. 3. 

 Hyria avicularis var. b. Lamarck, An. sans Vert., VI, 1819, 



p. 82. 



Quite variable in size, outline and degree of inflation as well 

 as color. In some specimens the posterior ridge is raised up 

 into a strong cord ; in others it is merely angular. In old shells 

 the Hgamental patch is large and distinct. 



Prtsodon compt.anatus (Hupe). 



Shell subrhomboid or subtrapezoid, somewhat inequilateral, 

 subcompressed or subinflated, slightly thickened; beaks not 

 greatly elevated; posterior ridge widely rounded, ending in a 

 blunt, rounded point near the base; dorsal outline nearly 

 straight, arched up a little in the middle ; winged in front and 

 behind ; anterior end rounded, narrowed and cut away below ; 

 base line curved; posterior end obliquely truncate, incurved; 

 epidermis brownish-black ; pseudocardinals strongly, vertically 

 striate ; nacre bluish-white, iridescent. 



Length 85, height 70, diam. 35 mm. 



Guiana. 

 Hyria complanata Hupe, Moll. Nouv., Ill, 1857, p. 80, pi. xv, 



fig. 3.— SowERBY, Conch. Icon., XVII, 1869, pl- iv, fig. 9. 

 Hyria synnatophora Kustkr, Conch. Cab. Unio, 1856, p. 141, 



pi. XLT, fig. 4. 

 Prisodon complanatus Simpson, Syn., 1900, p. 870. 



This species seems to dififer from the syrmatophorus in hav- 

 ing a much lower, widely rounded posterior ridge, which ends 

 below in a widely rounded point a little above the base of the 

 shell. The pseudocardinals are more strongly striate and it is 

 probably darker colored, as a rule, than syrmatophorus. 



