91 



a considerable space beneath the margin, on the in- 

 ferior side; the two teeth interlock when the valves 

 are closed. The substance of these shells is nacreous, 

 and their exterior covered with a brown or green epi- 

 dermis, the apices eroded or carious. They inhabit the 

 mud of rivers, placed with their apices downwards ; 

 some species have their valves slightly gaping; and 

 some produce fine pearls. 



rdinal Tooth short, not lamellar or substriated. 



Unio sinuata Unio carinifera 



elongata 



• • • Georgina 



crassidens 



• • • clava 



Peruviana 



• • • recta 



rariplicata 



• • ■ naviformis 



purpurata 



• • • glabrata 



ligamentina 



• • ■ nasuta 



obliqua 



• • • ovata 



retusa 



• • • rotundata 



rarisulcata 



• • • littoral is 



coarctata 



• • • semirugata 



purpurascens 



• • • nana 



radiata 



• • • alata 



brevialis 



• • • deladonta 



rhombula 



• • • sulcidens. 



