ACKNOWLEDGMENTS. 



THE author wishes to acknowledge the help which he has 

 received from his assistants, Mr. T. L. Duncan, Mr. H. 

 Cuzner and Mr. T. L. Erickson, in revising the manuscript 

 and in various other ways. 



Figure 49 is loaned by Professor John Gifford. 

 " 14 is loaned by Mr. H. B. Ayres. 



" 50 is from W. H. Rau. 



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