g.3 2$\ 



% \\\ju . -W, 



NOTE. 



It is but justice to Dr. Hagen to state that this monograph was 

 completed by him and handed to me for publication as early as 

 October, 1868. Circumstances over which I had no control have 

 delayed its passage through the press till now. 



LOUIS AGASSIZ. 



Cambridge, February 21, 1870. 



At 



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