IOI4 FRESH-WATER BIOLOGY 



*57 ( I 5^) Hinge heavy and strong; hinge plate wide and flat. 



Subgenus Plagiola s.s. 



The type and only species, P. securis Lea (Fig. 1519; X $), occurs abundantly in the Ohio and 

 Mississippi systems and south to Alabama. 



FIG. 1519. 



158 (157) Hinge delicate; hinge plate narrow. 



Subgenus Amygdalonaias Crosse and Fischer. 



A group of only three species character- 

 ized by the sharp posterior ridge and arrow- 

 shaped pattern of the epidermis. Two of 

 them occupy the Mississippi drainage, ex- 

 tending into southern Michigan and south to 

 Alabama and Texas. The third is peculiar 

 to Texas. Example, Plagiola elegans Lea 

 (Fig. 1520; X t). 



FIG. 1520. 



159 (160) Male and female shells alike; oval-solid, inflated, with a row of 

 large knobs running from the beaks to the center of the 

 base; hinge complete. Marsupium consisting of a few dis- 

 tinctly marked ovisacs situated just behind the center of the 

 outer gill and projecting far below the rest of the branchiae. 



Obliquaria Rafinesque. 



The typical and only species, O. reflexa 

 Raf. (Fig. 1521), ranges from Michigan 

 south to Alabama and Texas. 



FIG. 1521. 



