116 THE NAUTILUS. 



1. Subfamily: Margaritanin^. 



Gills without well-defined water tubes ; connections of the two 

 laminas by irregularly scattered prominences, but not by septa. 

 (This is a most remarkable character, in which Margaritana differs 

 from all other genera.) Supra-anal opening not separated from the 

 anal opening. Diaphragm (posterior part of gills, separating anal 

 and branchial openings) of peculiar shape : the outer lamina of the 

 outer gill is free from the mantle for a considerable distance. Inner 

 lamina of inner gills free from the abdominal sac. No papillae on 

 edge of mantle in front of branchial opening. (Marsupium and glo- 

 chidia unknown to the writer.) 



Genus and species : Margaritana margaritifera (L.). 



2. Subfamily : Unionin^. 



Gills with rather well-defined water tubes, the latter formed by 

 septa, which run the whole width of the gill, parallel to the gill- 

 filaments. Supra-anal ojiening not separated or (generally) separ- 

 ated from the anal, the closed part rather short. Diaphragm normal 

 (i. e., outer lamina of outer gills connected with the maiitle to their 

 posterior end). Inner lamina of inner gills always free from abdom- 

 inal sac. Ko papillae on mantle edge in front of branchial opening. 

 Marsupium formed by both gills or only by the outer gill; edge of mar- 

 supium always sharp (not distending). Water tubes not di.vided in 

 the gravid female. Glochidia semioval or semicircular, without 

 spine. 



1. Genus : Quadrula (Raf.) Ag. (restr.). Both gills (inner and 



outer) serving as marsupium. 

 Species : subrotunda (Lea) [and var. kirtlandiana (Lea)] — 

 ruhiginosa (Lea) [and var. trigona (Lea)] — pus- 

 tulosa (Lea) — metanevra (Eaf.) — cylindrica (Say) 

 — tritogonia (Ortm)' — undulata (Barn.). 



2. Genus: Rotundaria Eaf. (as subgenus). Outer gills only serv- 



ing as marsupium. Supra-anal opening not sep- 

 arated from the anal. Also with peculiar shell 

 characters. 



^ Quadrula tritogonia Ortm. ^ Tritogonia tuberculata (Barn.) of Simpson. 

 The nomenclature of this species remains to be settled. Since Qu. tul'erculata 

 (Raf.) is now removed from the genus Quadrula, the specific name tuberculata 

 might become available. Qu. lachrymosa (Lea) probably is also a true 

 Quadrula. 



