THE ANCESTRAL ORIGIN OF THE NORTH AMERICAN 
UNIONID/, OR FRESH-WATER MUSSELS 
By’ CHARLES A. WHITE 
The subordinal group of fresh-water mollusks, the Naiades, 
includes two recognized families, the Unionide and the Mutelidz. 
These two families have many essential characteristics in common 
and, together, they are distinctly separate from all other molluscan 
families. They are not only peculiar as regards certain portions 
of their structure and life history but, with few exceptions, also 
as regards the restrictions of their inhabitation. That is, the 
Naiades form the great group of mollusks which are commonly 
known as fresh-water mussels, all of which are confined to a fresh- 
water habitat and all will die quickly if immersed in salt water, or if 
removed to the land. This article is written with special refer- 
ence to the family Unionide, to the geographical distribution of 
its living representatives, and to the character and succession in 
time of its fossil representatives in North America. Therefore the 
Mutelidz, which are far inferior in numbers and variety to the 
Unionidez, and are confined to Africa and South America, will not be 
further referred to except in a general way. The following elemen- 
tary statements concerning the structure and physiological functions 
of the Unionide are given for the purpose of emphasizing certain of 
the facts which are to be stated concerning the integral survival of 
the family through long geological ages, its present separateness 
from other molluscan families and the wide geographical distribution 
of its living members; and also to illustrate the characteristics by 
which the fossil shells of the family are recognized as such. 
In a general way, the animal which produces, and is protected by, 
the shells of the fresh-water mussels is much like that of the common 
edible clam or, less closely, like the oyster. It is without a proper 
head, and also without some of the functional organs possessed by 
other animals; but it performs the function of locomotion, plowing 
slowly through mud and sand, by means of a muscular projection 
called the foot; that of respiration by gills, somewhat like those of 
fishes ; that of circulation by means of a rude pulsating organ which 
serves as a heart; that of digestion by a stomach; and that of repro- 
duction by minute eggs. The body, which consists of soft parts 
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