1895. PROCEEDINGS OF THE NATIONAL MUSEUM. 303 



cal, and may be applied to every genus, and that we may thus refer to 

 certain species as belonging to the group of Unio littoraHs, the grouj) 

 of Anodotita cygnea, the group of Spatha rubens, and the like. 



In 1817 Schumacher' subdivided Unio, and established the genus Mar- 

 garitana for the U. [Mya) margaritifer'ns of Linuieus, ou account of the 

 fact that it lacked the lateral teeth of the other species. Since that 

 time a number of North American forms have been added to this genus, 

 which has been quite generally acce])ted as such by modern authors, 

 among whom is Tryon;^ and as a subgenus b}' Lea ■ and Fischer.* 



After a good deal of study of the animal and shell, I am forced to the 

 belief that the different Margaritauas are merely a number of generally 

 not at all closely related species of Unios, in which the lateral teeth — 

 l)erliaps from various causes to be mentioned hereafter — have become 

 either more or less blurred or depauperated. Some of these, by the 

 characters of the shell and soft parts, evidently group with species of 

 Unios in which the teeth are nearly or quite perfect. In such species 

 as Margarltana rugosa, Barnes, ^1/. eonfragosa, Say, M. complanata, 

 Barnes, and 31. calceola, Lea, there are almost always more or less 

 perfectly develojjcd laterals which look as though they were diseased, 

 and have a blurred appearance, the normally single or double lamelhe 

 being divided into several irregularly developed, elongated ridges. 

 Nearly all the species occasionally have as perfect teeth as any CJnio. 

 The National Museum possesses a series of young shells of j1/. margari- 

 tifera (No. 00878) from the State of ^Vashingtou, in which most of the 

 specimens have fairly good laterals, and another specimen (No. 86280, 

 U. S. N. M.) in the Lea collection from Massachusetts has cardinals 

 and laterals as perfect as those of any Unio. The same is true of 

 many specimens of this species from Europe and northern Asia. The 

 group which this species typifies is a remarkable one, not only because 

 it shows great variation in the development of the hinge teeth, but for 

 its wide and somewhat peculiar geographical distribution. I place in 

 it the following species, beginning with those which have the laterals 

 least developed and proceeding to forms in which they are perfect: 



UNIO MARGARITIFERUS, Linn^us. 



All Europe; all northern Asia, including Japan; northwestern North 

 America south to latitude 40^ north; Upper Missouri River; Canada 

 and eastern United States south to latitude 40° north, in streams 

 draining into the Atlantic. Cardinals sometimes stump-like ami 

 imperfect; laterals generally wanting. 



'Essai d'un nouveau syst. des habit, des vers testacds, p. 137, 1817. 

 ^Structural and Systematic CoucLology, p. 240. 

 ^ Synopsis of the Uniouida', p. 67 et seq. 

 ■' Manuel de Conchy liologie, p. 1001. 



