IOI4 



FRESH-WATER BIOLOGY 



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



Subgenus Plagiola s.s. 



The tvpc and onlv ^pecie^ P sccuris Lea (Fig. 1519: X 2), occurs abundantly in the Ohio and 

 Mibbisbippi bvstems and south to \labama 



Fig. 1 5 19. 



158 (157) Hinge delicate; liinge plate narrow. 



Subgenus Aniygdalonaias 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 draina,ge, ex- 

 tending into southern Michigan and south to 

 Alabama and Texas. The third is peculiar 

 to Texas. Example, Plagiola elegans Lea 

 (Fig 15.-0; Xi). 



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. 



OUiquaria Rafinesque. 





ilYvif 



The typical and only species. 0. rcjhxa 

 Raf. (Fig. is2t), ranges from Michigan 

 south to Alabama and Texas. 



Fig. 1521. 



