FISHES OF GREAT 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 inhabits 
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 invade 
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. J 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, ft. F., “ European Animals,” p. 179. 
X Walker, Bryant, “ Distribution of Margaritana margaritifera,” p. 127. 
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