998 



FRESH-WATER BIOLOGY 



113 (114) All four gills serving as marsupia. Shell large, solid, rhomboid, 

 truncated posteriorly in the male, elongated, with a strong 

 posterior ridge, sexes dissimilar in shape, the posterior region 

 being rounded and subcompressed in the female; hinge com- ^| 

 plete; surface pustulose, except on the extended portion of 

 the female Tritigonia Agassiz. 



The type, T. tuber culata Bar . (Fig. 1485; X i), is very common in the Mississippi drainage and 

 in the Southern States from Alabama to Texas. 



Fig. 1485. 



