FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 111 



terlal had been collected. Taxonomic and relative abundance differ- 

 ences may be the result of incomplete quarrying at the site of sur- 

 face discoveries (one jaw on the surface may indicate fifty at six 

 feet down !) , or lithofacies differences may indicate a complete bio- 

 facies change, as suggested by Simpson and by Van Houten. The 

 final confirmation of one or more of the various possibilities rests on 

 the completion of exhaustive quarrying with improved collecting 

 techniques in the areas where the variegated, red-banded formations 

 are exposed and where the sample is essentially surface pickup at 

 present. 



During the last fifteen years several institutions in the midwestern 

 and western United States have been employing a screen -washing 

 technique for the recovery of small fossils. This technique calls for 

 the handling of tons of fosslliferous sediment. The sediment Is 

 washed through screen boxes. Available bodies of water are used, 

 whatever they may be: streams, lakes, stock tanks, or the ocean. 

 Fosslliferous matrix, frequently appearing quite barren on its surface 

 exposure, is quarried and shoveled into burlap bags for transport to 

 the washing site. Experience has shown that the transport distance 

 may be 20 miles or more before the method can be considered un- 

 profitable in terms of collecting expense. At the water, the matrix is 

 screen -washed, dried, and picked; and the number of specimens 

 recovered is often astounding when compared to standard quarrying 

 methods. During three summer seasons in the lower Eocene of north- 

 western Colorado and adjacent Wyoming, M. C. A^IcKenna and 

 colleagues of the University of California obtained about 20,000 

 individual specimens of teeth, jaws, and bones of ultra-small fossil 

 vertebrates. And these were taken from terrain that had been passed 

 over and considered unworthy for collecting by earlier workers. 

 Through a work period of five months, we have washed slightly more 

 than 30 tons of sandstone and have recovered 2,500 museum speci- 

 mens of fossil mammals from the late Cretaceous of Wyoming. This 

 is about one mammalian study specimen for each 24 pounds of 

 matrix. And these mammals are especially significant to the under- 

 standing of basic diversification (now largely conjectural) within 

 the Class Mammalia. Even greater numbers of specimens represent- 

 ing a combined assemblage of lizards, frogs, snakes, fishes, dinosaurs, 

 turtles, crocodiles, and birds were obtained from the depositional 

 association with the Cretaceous mammals. In both the Eocene and 



