idi4 



FRESH-WATER BIOLOGY 



157 (158) Hinge heavy and strong; hinge plate wide and flat. 



Subgenus Plagiola s.s. 



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

 Mississippi systems and south to Alabama. 



Fig. I sic 



58 (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 pecuHar 

 to Texas. Example, Plagiola elegans Lea 

 (Fig. 1520; XI). 



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, 0. reflexa 

 Raf. (Fig. 152 1 ), ranges from Michigan 

 south to Alabama and Texas. 



Fig. 1521. 



