NORTH AMERICAN SPECIES OF THE GENUS ATA? 247 



Mussel. 





Collected 



previous to 



1898. 



Per cent 

 infested. 



Collected 

 during 



1898. 



Per cent 

 infested. 



Unio occidens 

 TJnio ventricosus 



\ 



19 



100.00 



40 



100.00 



Unio luteolus 





60 



70.00 



33 



85.00 



TJnio ligament In ua 





57 



63.00 



75 



96.00 



Unio plicatus 

 Unio undulatus 



\ 



48 



64.50 



57 



84.00 



Unio gibhosus 

 Unio rectus 



I 



46 ' 



15.00 



36 



55.00 



Unio nasutus 





24 



4.00 









This past summer, however, in the case of U. spatulatus 

 and U. JYovi-eboraci, which are very similar in size and form, 

 an apparent exception to the rule was found; for of 46 speci- 

 mens of U. spatulatus, 20, or 43 per cent, were parasitized, 

 while of 24 of the other species collected with U. spatulatus, 

 all were infested. U. spatulatus has, to be sure, the ad_ 

 vantage in the angle at which the shells close and in activity, 

 but hardly sufficient to lead one to infer the above result from 

 simply a comparison of those characters. The mussels repre- 

 sented in the above table were all from Michigan, none from 

 elsewhere being included, in order to avoid any necessity of 

 allowing for a difference of locality. All were collected to- 

 gether along the Grand River at Grand Rapids, except U. 

 nasutus which was from Lake St. Clair and U. luteolus which 

 represented specimens from Lake St. Clair, Reed's Lake at 

 Grand Rapids, and White Lake, Muskegon County, together 

 with a few from North Michigan Lakes. Comparison of U. 

 luteolus from the different localities showed practically a uni- 

 form percentage. 



As between Anodontas and Unios, of 29 Anodontas taken 

 previous to this year at the same localities as the above men- 

 tioned Unios, omitting JJ. luteolus, 17 or 58.5 per cent were 

 infested; of those taken this summer 68 out of 74 or 72 per 

 cent were infested. Anodontas taken with the U luteolus 

 enumerated above, previous to 1898, showed a percentage of 

 72 per cent or 33 out of 46, and during 1898, of 95.5 per cent 

 or 21 out of 22. 



