508 mutelidjE. 



Genus TRIQUETRA, Klein. 



Outer gill united to the mantle as far as its extremity; 

 inner gill united to the foot. 



Shell equivalve, obliquely triangular, auriculate, the dor- 

 sal edge truncated and straight. Hinge with two little 

 elevated teeth ; the posterior or cardinal tooth divided into 

 numerous diverging portions (the inner portions being the 

 smaller) ; the anterior or lateral tooth very elongated and 

 lamellar ; ligament external and linear. 



Syn. Prisodon, Schum. Hyria, Lam. Paxyodon, 

 Schum. Hyriopsis, Conr. Triplodon, Sjpix. 



Ex. T. subviridis, Klein, pi. 120, fig. 1, 1, a. 



All the species of this genus known, are from the rivers of 

 South America. 



Species of Triquetra. 



Browuiana, Lea. elongata, Swains. 



corrugata, Lam. subviridis, Klein. 



Genus CASTALIA, Lamarck. 



Branchial aperture surrounded with cirrhi, anal aperture 

 smooth ; outer gill united to the mantle as far as its 

 extremity. 



Shell equivalve, inequilateral, trigonal ; beaks decorticated, 

 anteriorly recurved. Hinge with two lamellar, transversely- 

 striated teeth ; one posterior, remote, abbreviated, and some- 

 what tri-lamellar ; the other anterior, elongated, and lateral ; 

 ligament external. 



Syn. Prisodon, (3. Schum. Tetraplodon, Sj)ix. 



