ITYRIA 121 I 



A remarkable and beautiful Naiad, whicb is apparently 

 related to Tctral^lodon and Castallna, but cannot be referred 

 to either. The posterior truncation is more abrupt and the 

 posterior ridges are more sharp and elevated than in any 

 Unione that I am acquainted with. When the shell is held so 

 that the eye falls direct on the anterior dorsal portion the out- 

 line presented is that of a blunt wedge. 



Genus HYRIA Lamarck, 1819. 



Hyria Lamarck, An. sans. Vert., VI, 1819, p. 81. 

 Triplodon Spix, Test. Fluv. Bras., 1827, p. 35. 

 Ndia SwAiNSON, Tr. on Mai., 1840, p. 379. 



Shell subrhomboidal, solid, slightly inflated, narrower and 

 dorsally winged in front, and having a post-dorsal wing; pos- 

 terior ridge often double, causing the shell to be biangulate 

 behind ; beaks low, with strong, nearly radial sculpture, the 

 central bars coalescing below, the whole continuing as strong, 

 radial and zigzag ridges over more or less of the disk ; epider- 

 mis thick, greenish when young, brownish or blackish when 

 old ; there are two or more rather short, compressed pseudo- 

 cardinals in each valve, which are much split up into denticles ; 

 one lateral in the right and two in the left valve ; teeth often 

 somewhat vertically striated ; dorsal scars numerous ; nacre 

 whitish. 



Animal with mantle lobes imited together behind and fur- 

 nished with two short, contractile siphons. (J. E. Gray.) 



Type, Hyria corrugata Lamarck. 



In 1753 Klein (Testamen Methodi, p. 135, pi. 9, fig. 36) 

 used the name Triquetra for the shell, which has since been 

 .known as Hyria syrmatophora Gronovius. In 1817 Schu- 

 macher founded the genus Prisodon (Essai Nouv. Systeme, 

 p. 138), in which under section a he placed his P. obliquus, 

 a smooth species considerably drawn out, and in section b he 

 put P. truncatus, which is probably the same as the Castalia 

 ambigua of Lamarck. These certainly belong to two genera. 

 Dr. Lea in the Synopsis used Klein's name for the winged shell 

 (Hyria corrugata Lamarck), and used the name Prisodon for 



