476 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
PREVIOUS INVESTIGATIONS 
A first systematic search of the fossiliferous deposits was made by 
Gidley early in 1921, when, in cooperation with the United States 
Geological Survey and in the company of Kirk Bryan, he examined 
and collected from localities discovered by Bryan the previous year. 
The expedition proved to be of marked success, resulting in a collec- 
tion of materials that included specimens of exhibition value and a 
representation of faunas of considerable scientific interest, being the 
earliest to portray sizeable microfaunae of a relatively obscure por- 
tion of the Cenozoic. 
A popular account of the expedition (Gidley, 1922a) appeared in a 
report on the Explorations and Field Work of the Smithsonian In- 
stitution in 1921. Gidley’s preliminary report on the collection and 
detailed description of the rodents and lagomorphs were published 
by the United States Geological Survey in 1922, and in 1926 his 
report of the Proboscidea and Edentata appeared. C. W. Gilmore, 
in 1922, described a new turtle from the Benson occurrence, and in 
1928 and 1938 included statements on the lizard and snake remains 
from the Benson and Curtis Ranch localities, respectively, in his 
monographs on these groups. Remains of the rather large avifauna, 
principally from Benson, were studied by Alexander Wetmore (1924). 
The two faunas as a whole, or in part, have been the subject of 
much discussion in subsequent writings, including those of Hay 
(1927), Osborn (1936), J. R. Schultz (1937), and Wilson (1987); 
and new forms were added to the faunas by Frick (1937), Stirton 
(1931), and A. E. Wood (1935). 
On a second trip to the San Pedro Valley in 1924 Gidley made a 
collection from the Benson and Curtis ranch localities for the Ameri- 
can Museum under a grant from Childs Frick.- The materials in this 
collection, except for the heteromyid rodents examined by Wood, 
have not been described. Additional collecting in these beds has 
been done by parties for Mr. Frick, and the horned ruminant mate- 
rial therein was described by him (1937). 
In the company of E. L. Furlong, I visited the San Pedro Valley in 
1928, and together we made a small collection of mammal remains, 
including the type of Simonycteris stocki, from the Curtis ranch for 
the California Institute of Technology. Again in 1936 I was permit- 
ted to revisit the San Pedro Valley for the Smithsonian Institution, 
and the party of that season was successful in securing additional 
specimens of small mammals from both the Benson and Curtis ranch 
localities. A popular account of the 1936 expedition was included in 
the Smithsonian Institution report of explorations for that year. 
