THE MOLLUSCA 



1007 



138 (i35) Shell thin, triangular, greatly inflated, with a high, sharp posterior 

 ridge; pseudocardinals compressed, reflexed; laterals want- 

 ing Subgenus Bullella Simpson. 



This group is composed of two very peculiar species found only in South Carolina and Georgia. 

 Type, Alasmidonta arcula Lea (Fig. 1506; X I). 



FIG. 1506. 



139 (122) Shell small, thin, elongate-elliptical; beak sculpture consisting of 

 fine parallel ridges, looped up in the middle; a high, irregular, 

 compressed pseudocardinal in each valve; laterals nearly 

 or quite lacking Hemilastena Agassiz. 



The type and only species, H. ambigua Say 

 (Fig. 1507), occurs in the Ohio river system, 

 ranging north to Michigan, west to Iowa, 

 south to Arkansas, and east to Tennessee. 



FIG. 1507. 



140 (107, 121) Marsupium formed from the outer gill alone and usually from 

 the posterior portion only; edge of marsupium, when charged, 

 distending and bulging out beyond the original edge of the 

 gill; water tubes simple in the gravid female. Hinge com- 

 plete; male and female shells usually quite different. 



Subfamily LAMPSILINAE . . 141 

 Twelve genera: 141, 146, 151, 152, 153, 156, 159, 160, 161, 162, 163, 164. 



