228 



R. W. PENNAK 



pods, and although they are usually considered more typical of the 

 Great Plains, they are just as characteristic of suitable mountain and 

 desert habitats west of the Divide. 



Table I. Systematic and Zoogeographic Status of Certain Groups of 

 Freshwater Invertebrates in the Western Portion of the United States 



Certain of the generalizations thus far discussed are summarized 

 in an alternate fashion in Table I. The twenty-one taxa in the 

 first column are listed in order of their decreasing relative species 

 abundance in the West (particularly as compared with the situation 

 in the eastern half of the United States). The first five taxa comprise 

 a group characterized by a preponderance of cosmopolitan species, 



