EXPLANATION OF PLATE IV. 



Fig. 9a. — Right gills of Fusconaia undata Irigonoides Frierson, showing 

 both marsupial, — a Unionine character. 



Fig. 9b. — Diagrammatic, vertical section of a Unionine ovisac at 

 ventral margin. 



Fig. loa. — Right gills of Lastena suborbiculata (Say) showing entire 

 outer one marsupial, — an Anodontine character. 



Fig. lob. — Diagram of Anodontine ovisac showing lateral water tubes 

 as provision for aeration of embryos. 



Fig. iia. — Right gills of Proplera alata (Say) showing only posterior 

 portion of outer one marsupial, — a Lampsiline character. 



Fig. lib. — Diagram of Lampsiline ovisac, showing rupture of ventral 

 edge to allow escape of glochidia. 



