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ANNALS NEW YORK ACADEMY OF SCIENCES 



puzzling admixture oi' characters, which makes it doubtful whether they 

 should be reckoned as pertaining to the same stock as the other hystrico- 

 morphs. The remaining families, while chiefly South American, are 

 also partly represented in the Ethiopian, Oriental and Holarctic regions. 

 It may be possible, in view of the facts that the European Tlicridomyidae 

 antedate geologically any specialized hystricomorphs, are apparently di- 

 rectly intermediate between the primitive rodent type {Parann/s and its 

 allies) and the hystricomorphs and show the early stages of differentia- 

 tion of several hystricomorph families, that the Ilystriconiorplia are a 





T'ertiary ancestors 

 of Eret/iizon t/dae 



Fig. 14. — Distribution of the true porcupines (llystriculw) and Scic ^Vurld porcupines 



(Erethisontidw) 



The HystrlcldiB appear to be of Palaparcfic dispersal, the Erethizontldae are apparently 



of Neotropical origin. 



group of Holarctic origin which has spread into all the southern conti- 

 nents and specialized independently on parallel lines. But their entire 

 absence from the recorded North American Tertiary is then explainable 

 only by the defective record, and our knowledge of Xorth American Ter- 

 tiary rodents is so extensive that I should hardly regard this assumption 

 as justifiable. The fact that the highest and most specialized types are 

 South American necessarily involves the idea that that continent has 

 been the most important center of their later development and dispersal. 



