90 



transversely striated, but not extending to the upper 

 margin, and covered with a brown epidermis. 



This shell appears to inhabit fresh waters, and indi- 

 cates that the Trigone&e form a transition from the 

 Arcaceae to the Naiades. 



Castalia ambigua. ' 



UNIO. 



MYA MARGARITIFERA. 



■Linn. 



Wood's Conchology, plate 23, fig. 1, 2, 3. 

 Linn^us included this genus in the G. Mya, which 

 consists entirely of marine shells, while the true Uni- 

 ones are fresh water shells, and quite distinct from 

 them in their form, hinge, and position of the ligament. 

 The external appearance of the Unio greatly resem- 

 bles the G. Auodon, and is nearly allied to it in many 

 other respects, but it attains a much greater solidity, 

 and the formation of the hinge is most clearly different, 

 having a short cardinal tooth on each valve, generally 

 single on the left side, and divided on the right into 

 two lobes, with a lengthened lateral tooth, compress- 

 ed, canaliculated, extending along, and occupying 



