GENERA OF SHELLS. 35 



t 



TRIGONIANA. 



Cardinal teeth lamellar, transversely, striated. 

 The teeth of the trigoniana differ from those of the 

 arcacea by the stria? being on separate laminaB, instead 

 of being on the hinge itself. 



TRIG ONI A. Shell equivalve, inequilateral, 

 triangular, sometimes suborbicular. Cardinal 

 teeth oblong, flattened at the sides, divergent, 

 furrowed transversely ; two are on the right 

 valve, furrowed on both sides, and four on the 

 other, furrowed on one side only ; ligament 

 exterior, marginal. 



The trigonise are regular, free, very inequilateral sea 

 shells, and all, except one species, only known in a 

 fossil state, fourteen of which are enumerated. 



T. pectinata. 



CAST ALIA. Shell equivalve, inequilateral, 

 triangular; beaks decorticated, recurved pos- 

 teriorly. Hinge with two lamellar teeth, stria- 

 ted transversely ; one posterior, remote, abbre- 

 viated, sub-trilamellate, the other anterior, 

 elongated, lateral ; ligament exterior. 



The castalia resembles a trigonia in appearance, but 

 differs from it in the number and situation of its teeth, 

 which have more resemblance to those of the unio, 

 between which and the trigonia it seems to be inter- 

 mediate. It appears to be a river shell. 



C. ambigua. 



