332 TBE TEABLY FEESn-WATER MUSSELS— SIMPSON. vol. xvm. 



soutberu Virginia and even into Georgia. The Red River of the ]S^orth, 

 the Mackenzie, the (Ireat Lakes, most of the lower peninsula of Michi- 

 gan, and the southernmost portion of Canada are, for the most part, 

 l^eopled with Mississippi Valley species. 



Ko equal area on earth has such a diversity of Xaiad life or such 

 magnificent shells. Here are found the largest species in the world; 

 here are forms with knobs, pustules, angles, lobes, and concentric sculi>- 

 ture. The nacre of many of them is wonderfully rich in tints of silver, 

 pink, purple, salmon or red, and it is equaled in beauty by the elegant 

 patterns of external painting, in stripes and mottlings and delicate hair 

 lines. Perhaps twenty or more species of this region are extralimital, 

 and about half as many from other areas occur within its borders. 



The Atlantic Region. — East of the Appalachian chain, and occupying 

 all the rivers and streams from Florida to Labrador that drain into 

 the Atlantic, there is a set of Unionids, consisting of Unios and Ano- 

 dontas, generally moderate in size, thin in structure, and for the most 

 part without strong angles, sculpture, or brilliant coloring. Toward 

 the southern part of this region the forms are immensely variable and 

 puzzling, and I do not know of iiwj other area in the M^orld in which 

 it is so diflicult to satisfactorily separate species and groups. Although 

 both in the southern and northern part of this province the forms of the 

 Mississippi Valley have entered freely, until they have met and over- 

 lapped, yet there are perhaps not more than one or two species which 

 belong in this region or members of any of its groups that appear in the 

 waters of the Mississippi drainage proper. AnodontaJ'r<i</ilis, Lamarck, 

 a form characteristic of the Atlantic in-ovince, is found in several pla(;es 

 in the Mississippi ai-ea, notably in Minnesota; and Into r/tdiatiis, 

 Lamarck, is doubtfully reported from the St. Croix River, Wisconsin. 



The specimens of Anodonta footiana, Lea (another northern form), 

 said to come from the Illinops River, are no donht Anodonta orata, Lea, 

 There are scarcely a dozeu Mississippi drainage species found within 

 this region. 



The Neotropieal Region. — The entire continent of South America 

 forms a single region of ii^aiad life, containing four genera of Unionidaj 

 (Unio, Frisodon., Tetraplodon and Gastalina) and six of Mutelidaj 

 {Glaharis, Leila, 3Ionocondylcea, Fossula, Iheringella and Mycetopoda). 



The Unios are generally oval or rounded, moderate in size, usually 

 slightly sulcate, and covered with a uniform brownish or greenish brown 

 epidermis. All have radial beak sculpture, and very few have any other 

 than what I have mentioned. 



The genus Unio is represented throughout the entire area, and 

 strangely enough the great Andean chain does not form a barrier 

 between groups. The assemblage typified by the well known oval, 

 compressed Unio ellipticus, Spix, seems to be scattered over this whole 

 area, and species belonging to this group in Peru and Chile on the 

 Pacific Slope of the continent can scarcely be said to diifer from forms 



