466 SINCLAIR— THE " TURTLE-OREODON LAYER." 



for with every spreading eastward of the streams, due to cHmatic 

 causes, the fauna Hving along the water courses would recur in the 

 channels cutting the clays in which we find remains of the animals 

 of the plain. So far as my observations have gone, these channels 

 aic posterior to the formation of the caliche nodules and not con- 

 temporary with them, since they cut out the nodular zones. 



That these changes did not involve a lowering of temperature is 

 suggested by the presence of alligator-like forms, few in numbers, 

 but occurring, nevertheless, in the Titanotherium beds.^ They per- 

 sist in the higher plains area until Pliocene time, as shown by the 

 discovery of a single crocodilian vertebra in the Snake Creek Plio- 

 cene gravels in Sioux County, northwestern Nebraska, by our 1914 

 expedition.'^ I have not yet identified any volcanic ash in the " red 

 layer," nor have I noted therein any sediments which might be re- 

 garded as of seolian origin. 



Plate VII. 



Fig. I. Rusty concretions and turtles weathering out of the " turtle-oreo- 

 don layer " which covers the flat in the foreground. Princeton collecting 

 locality lOisDaa on the west side of the Reservation road, 5.1 miles south 

 of Scenic, Pennington County, South Dakota. Looking north. This local- 

 ity is near the divide between Cheyenne R. and White R. drainage in the 

 Bear Creek basin. Typical exposure at a highly fossiliferous locality. 



Fig. 2. Contact, indicated by the asterisk, between the Titanotherium beds 

 (foreground) and the banded Oreodon beds (background) on the west side 

 of Hart Table, valley of Indian Creek, Pennington County, South Dakota. 

 Looking east. The slight southeasterly dip of the Oreodon beds is also 

 shown. Princeton collecting locality 1015A i. 



^ F. B. Loomis, " Two New River Reptiles from the Titanothere Beds," 

 Am. Jour. Sci., 4th Series, Vol. 18, pp. 427-432, 1904. M. G. Mehl, " Caiman- 

 oidea Visheri, a New Crocodilian from the Oligocene of South Dakota," 

 Jour, of Geo!., Vol. XXIV., No. i, Jan.-Feb., 1916, pp. 47-56. 



7 Proc. Am. Phil. Soc, Vol. LIV., No. 217, May-June, 1915, p. 77. 



