Bull. U. S. B. F., 1919-20. 



Plate XIII. 



[See text, pace 130. and compare Plate XXI, tic i, 

 showing marsupium occupyint: outer gills only. 

 Figures after Lcfevre and Curtis.) 



-Niggerhcad mussel, Quadiula ebenus. 

 marsupium occupying all four gills. 



■^wA'-^iJA*^JAi*■Xv-^^ 



Fig. j. — Black sand-shell. Lampsilis rccla; marsupium 

 occupying only posterior end of outer gills. 



Fig. 3.— Three-homed -uarty-back. Obliquaria rc^eara; marsupium 

 occupying middle region of outer gills. 



Fig. 4. — Dromedary mussel, 

 Dromus dromas; marsupium 

 occupying only lower border 

 (»f outer gills. Anterior end of 

 gill not included in marsu- 

 pium but overhangs it. 



Fig. 5.— Kidney-shcU. Pi_\\hol>ru/iriiit.\ phascolu^. mariupmm occupyuig entire lower 

 border of outer gills and much fulded. 



