THE MOLLUSCA 



1007 



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

 ridge; pseudocardinals compressed, reflexed; laterals want- 

 ing. . Subgenus Bidlclla Simpson. 



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

 Type, Alas mido Ilia arcula Lea (Fig. 1506; X t)- 





Fig, isotj, 



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. . Uemilastena Agassiz. 



The type and only species, H. ambigua Say 

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

 ranging north to ]\Iichigan, 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 Lami^silinae . . 141 

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



