﻿ANOnONTA 359 



evenly curved, narrow hinge line, zi'liicli is never incurved in 

 front of tlie beaks. The marsupium occupies the entire outer 

 gills, forming- thick, smooth, generall}- dark colored, pads. 

 The genus is distributed throughout all or nearly all of North 

 America and the Paljearctic region, and is quite abundant in 

 the northern part of the Oriental region. Fischer made the 

 subgenus Pteranodon, founded on these oriental forms, but I 

 see no reason for giving them a higher rank than that of a 

 group. I have examined the gravid females of specimens of 

 these oriental forms and they scarcely differ from those of 

 the Palaearctic or American species. The forms I have placed 

 in Strophitus and Anodontoides differ from the true Anodontas 

 in beak sculpture, in having rudimentary teeth mid in having 

 the hinge line incurved in front of the beaks. There are also 

 dififerences in the anatomy, which are quite important. 



Group of Anodonta cygnea. 



Shell very evenly rounded in front, pointed behind, the 

 point elevated above the base, more or less winged on post- 

 dorsal part, the line from the posterior part of the wing to 

 the hinder point usually incurved ; beaks flattened, the sculp- 

 ture consisting of numerous more or less concentric ridges, 

 which are sometimes broken up into rather irregular corruga- 

 tions. 



Animal having the inner gills the larger, free from the ab- 

 dominal sac nearly or quite their whole length ; palpi large. 



Anodonta cvgnka (Linnreus). 



vShell long elliptical, oval or subrhomboid, thin or somewhat 

 subsolid, moderately inflated, inequilateral, the beaks and the 

 area near them normally compressed ; in some forms the beaks 

 are rather full but never high nor sharp ; their sculpture con- 

 sists of numerous, rather closely set, fine, low ridges, which 

 either follow the growth lines or are slightly corrugated, this 

 sculpture extends well out on the shell ; anterior end evenly 

 curved from the region of the beaks around to the base ; 

 behind the beaks there is usually a more or less developed dor- 



