UNIO. 



Lam, 



TESTA SBquivalvis, inaequilatera, transversa, 

 epidermide olivacea induta. Impressiones 

 musculares duae, postica composita. DenteSj 

 cardinalis unicus, brevis, irregularis, simplex 

 aut bipartitus, substriatus; laterales duo, elon- 

 gati, compressi, antiee producti, nonnunquam 

 obsoleti. Ligamentum externum. 



A Genus of fresh water shells placed amon^ the Myae 

 by Linne, but nevertheless well discriminated by its 

 peculiar characters; and, perhaps, forming one of the 

 most natural genera, intermediate, as we believe, be- 

 tween the Lamarckian Anodonta (Anodon) and Hyria; 

 which latter may, we think, be very properly disunited 

 from it as forming a well characterized Genus, distin- 

 guished from Unio by its linear posterior teeth, and its 

 two wings. We are compelled, however, to unite La- 

 marck's Castalia to Unio, not indeed without offering the 

 tribute of our deserved commendation to that great Con- 

 jcholpgist, for his discrimination in placing this latter 

 Genus between his Trigonice and his Naiades: for we can- 

 not suppose that he considers the Trigonice as at all con- 

 nected with the Arcacece, much less with the Nucula, 

 whose ligament is internal ; next to which, however, we 

 find it placed. 



The Uniones are mostly transverse, sometimes almost 

 cordate, and even suborbicular, inequilateral, equi valve, 

 mostly thick, sometimes extraordinarily so, though in 

 some few instances rather thin shells 3 generally covered 



