89 



on both sides, and in the other, four are grooved only 

 on one side ; ligament exterior and marginal. 



Another recent species of this very rare shell ex- 

 ists in the late Earl Tankerville's splendid cabinet. 

 Trigonia pectinata. 

 [There are fifteen fossil species.] 



CASTALIA. 



In the cabinet of the Marquis de Dree 



LaMarck having seen only one species of this singu- 

 lar fresh water shell, has given it as the type of his ge- 

 nus Castalia; it greatly resembles the G.Trigonia, but 

 the lamellar teeth are different in number and position, 

 and more like those of the G. Unio; it cannot however 

 properly be classed with either, as it occupies an in- 

 termediate station between the two. Shell equivalve, 

 inequilateral, subtriangular, apices eroded, and curved 

 towards the posterior side; hinge with two lamellar 

 teeth, transversely striated, one of them posterior, dis- 

 tant, shortened, and subtrilamellar; the other anterior, 

 lengthened and lateral: ligament exterior. The sub- 

 stance is nacreous: valves with longitudinal flat ribs, 



