vi INTRODUCTION. 



Of the sub-genus Margaritana there are 14 admitted spe- 

 cies, and 2 which are unknown to me. 



Of the sub-genus Dipsas I know of but two species, both 

 of which are recent. 



Of the sub-genus Anodonta there are 50 admitted species, 

 and 6 which are unknown to me. Of fossil species there is 

 one which is doubtful. 



Of the sub-genus Pleiodon there is but one species, and 

 that recent. 



The genus Iridina has 7 species, all recent. 



