126 D. E. SAVAGE 



fiable bones and bone fragments. This statement is trite unless 

 considered in the Hght of the actual case history of fossil vertebrate 

 collecting. 



6. It is possible that there may be a fixed relationship between 

 mammalian faunal facies and the containing lithofacies, but pre- 

 vious generalizations as to this possibility were premature and will 

 have to be confirmed by exhaustive quarrying and by improved 

 collecting methods. 



7. Screen washing is being applied to formations that earlier 

 workers believed to be unprofitable for recovery of fossils. The 

 abundance of small vertebrates in these formations indicates that 

 we may obtain large samples from seemingly barren, red, red- 

 banded, or varicolored flood-plain deposits. 



8. Evidence is strongest for the Nearctic origin of Marsupialia, 

 Edentata, Tillodontia, Taeniodonta, Perissodactyla, Dermoptera, 

 Primates, and Rodentia, but the evidence is not conclusive. These 

 orders evidently differentiated within the Infraclass Eutheria in the 

 interval extending through late Cretaceous and Paleocene, roughly 

 85 to 65 million years ago. 



9. About 35% of the living land mammal families of Nearctica 

 are autochthonous, about 7% endemic; most of the families origi- 

 nated in the interval late Eocene-Oligocene, approximately 50 to 35 

 million years ago, 



10. About 70% of the modern Nearctic land mammal genera are 

 probably autochthonous; and as many as 15% were living as early 

 as late Miocene, 30% by middle Pliocene. All together, the genera 

 originated from about 15 million to possibly several thousand years 

 ago. 



11. About 97% of the modern Nearctic land mammal species are 

 probable autochthons that originated in the interval, later Pleisto- 

 cene into Recent, possibly two or three hundred thousand years 



ago to present. 



REFERENCES 



Axelrod, D. I. 1940. The Mint Canyon flora of southern California: a 



preliminary statement. Am. J. Sci., 238: 577-585. 

 . 1950. Evolution of desert vegetation in western North America. 



Carnegie Inst. Wash. Publ., 590: 215-306. 

 . 1952. Variables affecting the probabilities of dispersal in geologic 



time. Bull. Am. Museum Nat. Hist., 99: 177-188. 



