GILMORE: the fossil turtles of the UINTA FORMATION 103 



likely that a species known from a lower horizon continues over into a higher 

 horizon/ therefore a hunt for characters to separate it from the other species of 

 the genus is instituted, with the result that minor differences are magnified to 

 represent specific differences, when, had the specimen come from a formation in 

 which species of the genus were already known, it would in all probability have 

 found a resting-place within one of the described species. 



In order to facilitate comparisons of the descriptions here given with those of 

 other described forms I have closely followed the order of arrangement used by Hay 

 in his monographic study of the fossil turtles of North America. 



At this point I wish to acknowledge the assistance rendered me while this 

 paper was in the course of preparation. First of all I express my gratitude to Dr. 

 W. J. Holland for his hearty cooperation at all times, for the privilege granted me 

 of studying this fine collection of fossil turtles, and for his editorial oversight of 

 the work. I am under obligations to Dr. W. D. Matthew, of the American Museum 

 of Natural History, New York, for the loan of type-specimens, and to Dr. 0. P. 

 Hay, to whom, because of his wide knowledge of the turtles, I am especially in- 

 debted for invaluable advice upon numerous occasions. The text-figures were 

 made by the well-known artist, Mr. Rudolph Weber, the photographs are by Mr. 

 Arthur Coggeshall, of the Carnegie Museum. 



Geological Occurrence. 



All of the specimens considered in the present paper are from the Uinta forma- 

 tion as exposed in the Uinta Basin at the southern base of the Uinta Mountains, 

 and from that part of the basin which lies within Uinta County, Utah. The 

 geological positions of the various specimens as here given were taken from the 

 original field-labels which accompanied each specimen, so that these determina- 

 tions are wholly the work of Messrs. Earl Douglass and 0. A. Peterson, whose long 

 experience in the field insures the accuracy of their observations. 



In 1895 the Uinta formation was divided by Peterson' into three levels, or 

 horizons, designated as follows, A (Lower), B (Middle), and C (Upper) Uinta. 



The remains of turtles have now been found in all three horizons, though 

 judging from the present collection, individuals occur most abundantly in Horizon 

 B, but the number of species recognized in the collection is about evenly divided 

 between Horizons B and C. Up to the present time only one species is known from 

 Horizon A. Douglass-' has pointed out that "the lower portions of these deposits 

 may be, and probably are, contemporaneous with portions of deposits in the 



2 Peterson, 0. A., Bull. Amer. Mus. Nat. History, VII, 1895, p. 74. 



3 Douglass, Earl, Bull. Geol. Soc. of America, vol. 25, 1914, p. 418. 



