FISHES OF GEEAT LAKES 51 



the silver chub (Semotilus corporalis), which only frequents 

 clear streams, could have spread in post-Glacial times from 

 the south-eastern counties to the St. Lawrence basin. Two 

 more examples might be mentioned that are practically 

 confined to the drift area and they are among the most 

 interesting geographical puzzles of North America. 



The first of these is the mud minnow (Umbra limi), a 

 small minnow-like fish frequenting muddy and reedy ponds 

 near the Great Lakes as far east as Quebec. A closely allied 

 species lives in streams and ponds in the eastern States from 

 Connecticut to South Carolina. The only other relative of 

 these two little fish, in fact the only other member of the, 

 family of mud-minnows (Umbridae), is Umbra krameri found 

 in Hungary. Dr. Gill* gives a most interesting account 

 of their habits. 



Blanding's pond tortoise (Emys blandingi) is the second 

 example I have in my mind. It is a small species, the black 

 carapace or shell being dotted all over with numerous yellow 

 spots. It is entirely confined to the drift region from 

 Wisconsin in the west to New Hampshire in the east. It 

 agrees with Umbra in so far as its only relation inhabijts 

 Europe. The European species (Emys orbicularis) is 

 common in central and southern Europe. A map of its range 

 is given in my work on the European animals. f 



A more striking instance of a case of survival within the 

 glaciated, or drift area, is afforded by the fresh-water pearl- 

 mussel (Margaritana margaritifera). Being an immensely 

 ancient species which no doubt originated in North America, 

 it has taken advantage of old land connections to invaxle, 

 Europe and eastern Asia. In North America it occurs within 

 the ancient eastern land-mass in several localities, notably 

 in Labrador, on Newfoundland and Anticosti Islands, in 

 the Quebec province and in New England.^ From its western 

 station in the lower Saskatchewan it is separated by a 

 tract of over one thousand two hundred miles in which 

 it is unknown. This discontinuous distribution implies 



* Gill, Theodore, " The Umbras." 



t Scharff, E. F., " European Animals," p. 179. 



| Walker, Bryant, " Distribution of Margaritana margaritifera," p. 127. 



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