594 FOSSIL UNIOS TOKONTO SIMrSON. 



pro))al)ility by a now submerged laiulway, and to-day is found iu British 

 Columbia, Wasliington, Oregon, northern California, and in the upper 

 waters of the Missouri. It is again met with in eastern Canada, New 

 England, Pennsjivania, and New York, but has not been reported from 

 any of the intervening territory. Tlie suggestion made several years 

 ago by Prof. A. G. Wetherby * that it had been destroyed in this region 

 by glaeial action seems the most reasonable, and it is ])ossible that at 

 the eastern side of the continent it might have survived in the area not 

 covered by the ice cap or that it may have been driven to the south- 

 ward before it. This is the only naiad now found living within the 

 Mississippi drainage area that may said to belong to the Atlantic sys- 

 tem, and it is undoubtedly an immigrant. It probably entered the 

 Missouri through streams which connected that river with the North- 

 western lake system. 



Unio radiatiis, a characteristic Atlantic drainage form, has been 

 reported from Lake Winnipeg and the Nelson Kiver,t but it ap- 

 proaches so near to Unio luteolus, a common Mississippi shell, that the 

 identification may be considered somewhat doubtful. Unio compla- 

 7iatus^ another characteristic Atlantic area species, has, on the excellent 

 authority of Mr. Bryant Walker, been found in the southern peninsula 

 of Michigan, and Unio niitiutns, a third abundant aiul M'idespread east- 

 ern form, is frequently met with in that state, and in streams in 

 northern Ohio that drain into Lake Erie. But the Red River of the 

 North, so far as is known, is peopled wholly with Mississippi Valley 

 Naiades, and some of them extend to the Mackenzie Kiver. 



At the time during the Champlain period when the waters of the 

 northern lake region overflowed into the Mississippi Basin, many of 

 the hardier, more vigorous, and characteristic species of the latter ter- 

 ritory migrated northward and established themselves; most of them 

 remain in the streams that now drain northward and northeast- 

 ward, but a few have jwssibly retreated, while others, including three 

 of those received from Toronto, are to-day in all probability confined 

 to the Mississippi Valley. The lower peninsula of Michiganis almost ex- 

 clusively inhabited by these forms, as well as the Great Lakes, and they 

 extend well down the St. Lawrence and north and east into Canada. 



To briefly recapitulate, then, the ITuio fauna of the Mississippi Val- 

 ley is remarkably distinct, being nearly related only to a part of that 

 of northeastern Asia. It is an old fauna, dating back through an 

 almost unbroksn series of species to the Laramie group of the Creta- 

 ceous, and it is remarkably developed in large, vigorous species and 

 numerous individuals. That these forms are dominant is proven by 

 the fact that they so exclusively occupy this vast area, and that they 

 have spread so widely into other regions, through a great variety of 

 climate and conditions. 



* .11. Ciu. Soc. Nat. Hist. .Inly, 1881, p. 7. 



t ]>and aud fresh-water shells of Manitoba. Kolxrt Christy. 



