FRESH-WATER MUSSELS. 



95 



the stronger development of the hinge teeth which aid in keeping the two valves of the 

 shell in perfect apposition. 



Since a river presents from source to mouth conditions of varying suitability for any 

 form of animal life, there will usually be found in some measure a longitudinal succession 

 of mussels. Shelford (1913, p. 122) gives a table showing the longitudinal sequence of 

 eight species of mussels in the Calumet Deep River. 



If one goes down a river from its headwaters, making collections of mussels at 

 various points, many species may be found at each place, but some species first encoun- 

 tered may disappear before the upper waters are passed. Others appear here or there 

 and perhaps disappear as one proceeds still farther down. The mussel fauna of the 

 different sections of the stream are characteristic, although one or more species may be 

 so adaptable as to live throughout the entire course of the stream. 



This longitudinal succession of species is well illustrated by Table 2, which shows 

 the distribution of mussels in the Grand River, Mich. 



Table 2. — Longitudinal Distribution op Mussels in Grand River, Mich." 



o Observations by R. E. Coker in 1909. 



Summary of Table. 



Total species observed 26 



Other species occurring above Portland 6 



Species occurring throui;hout river 10 



Species found only at or below Portland 10 



Species not found below Grand Rapids , _ 4 



Species found only below Grand Rapids 6 



It will be observed at once that a far greater number of species is found in the lower 

 part of the stream. Thus, while only lo of the 26 species obser\^ed in the river were 

 found near the headwater lakes, 22 species were met in the section of the river between 



