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. ... . Triligonia Agassiz. 



The type, T. lubenulalaBar. (Fig. 14S5; X s)- 's very common in the Mississippi drainage and 

 in the Southern States from Alabama to Te.\as. 



Fig. 1485. 



