00 MICHIGAN ACADEMY OP^ SCIENCE. 



ditions of unione life liave been preserved unbroken notwitlistanding the 

 physical changes that have taken place during the Mesozoic and Tertiary 

 periods, it was doubtless accomplished througli streams that are now 

 western tributaries of the great Mississippi river system and which were 

 then outlets of those great lakes in the deposits of which the fossil 

 Unio7iidw are now found." By this means an entrance was afforded 

 into the waters of the Mississippi valley and an escape from the extinc- 

 tion which overtook their ancestors upon the final dessication of the 

 Laramie sea. That this invasion into the Mississippi valley took place 

 at a very early date is shown by the fact that the immigrants had time 

 under their changed environment to develop into the species as they now 

 exist before the advent of the glacial epoch. The extension of these forms 

 through the eastern portion of the continent, both in preglacial and 

 recent times, has been limited only by the physical barriers caused by the 

 great water-sheds lying between the Mississippi, St. Lawrence and At- 

 lantic drainage systems and unfavorable climatic conditions toward the 

 far north and south. 



Of their preglacial range to the north nothing is now known. But their 

 hardy nature and ability to extend into new territory is shown by the 

 fact, that during the temporary recedence of the great glacier, certain 

 species found their way north through the then existing southern outlet 

 of Lake Michigan into the St. Lawrence valley as far east, at least, as 

 Toronto and were subsequently extinguished by the return of glacier. 

 Upon the final disappearance of the glacier, but before the present drain- 

 age of the St. Lawrence system to the east had been established, a second 

 immigration took place whose descendants, all possible retreat to the 

 south having been cut off, now peo])le the lakes and rivers of the states 

 bordering on the great lakes. To the east, the Appalachian mountains had 

 proved an almost total barrier at all times to any general extension in 

 that direction. 



The relations, if any, which the peculiar fauna now found in the states 

 east of those mountains bear to the Mississippi valley, fauna, have not 

 yet been worked out. There is reason to believe, however, that it may be 

 genetically connected through some early migration which spread around 

 the southern end of the range and thence northward along the coast, 

 and has been enabled to develop its characteristic form under peculiar 

 conditions of isolation and local environment. 



The peculiar and exclusive fauna of California has already been alluded 

 to, but this characteristic is nowhere more strangely emphasized than 

 in respect to its Unionidw. There is absolutely no relation whatever 

 with the eastern forms, and there is not the slightest evidence that a 

 single one of the multitudinous unione inhabitants of the Laramine sea 

 ever succeeded in passing over the mountains into the low lands of the 

 coast. As has already been stated, the entire fauna of this region con- 

 sists of a single European Margaritina and a few Anodontw, which are 

 so closely related to existing European forms that their specific dis- 

 tinction is very doubtful. And thus again, and most unmistakably, is the 

 theory of the foreign origin of the fauna of the Pacific coast substan- 

 tiated by the undeniable facts of the present characteristics and distribu- 

 tion of its molluscan inhabitants. 



