Mylodon harlani, from Rock Creek, Ttxas. 329 



These, together with the Mylodon, make at least twelve asso- 

 ciated species representing five families of mammals and one 

 of reptiles. 



Geologic age. — The age of the fauna has been set forth by 

 Osborn,* who thus speaks of the Rock Creek beds : 



" These beds are extensively exposed in the Staked Plains 

 of Texas along the south side of Tule Canon. As described 

 above (p. 362) they represent a Lower Pleistocene river channel 

 cutting its way into an older Miocene horizon." 



And on page 362 he says : 



"A third period of river or flood-plain formation traversing 

 the same Lower Miocene substratum [of northwestern Texas] 

 occurred during the Pleistocene epoch, laying down the broad 

 bands of the ' Pock Creek formation', also of fluviatile, allu- 

 vial, and seolian origin, composed of cross-bedded sands, 

 gravels, and clays. The wind, carrying large quantities of fine 

 dust and sand on the surrounding plains, may have played an 

 important part in forming these deposits. The mammals 

 represented consisted wholly of land forms, and some of the 

 bones show weather-checking ; they contain the characteristic 

 Lower Pleistocene forms, Eguus, Elephas imperator, and 

 Platygonus" 



Doctor Hayf in 1912 says : 



"In his Age of Mammals, 1910, Osborn adopts, in general, 

 the views of Cope, recognizing, however, four faunas, which 

 he named the fauna of the first or Equus-Mylodon zone ; the 

 fauna of the second, or Megalon}-x zone ; the fauna of the 

 third, or Ovibos-Rangifer zone ; and that of the fourth, or 

 Cervus zone. The latter fauna is that which existed at the 

 time of the discovery of the continent by Columbus. . . . Pro- 

 fessor Osborn holds that the Equus-Mylodon fauna occupied 

 the dry Plains regions, but also the coast of Florida (p. 452), 

 while the Megalonyx fauna had possession of the forested 

 regions of the eastern part of the United States and of the 

 Pacific Coast (p. 467). . . . 



" As to the time of the existence of the Equus-Mylodon and 

 the Megalonyx faunas, Osborn (p. 454) is inclined to believe 

 that they were to a great extent contemporary, but that 

 probably early phases of the Equus-Mylodon fauna antedated 

 the beginning of the Megalonyx fauna." 



Thus the evidence points to the age of the species under 

 consideration as early Pleistocene. 



* Osborn, H. F., The Age of Mammals, p. 458, 1910. 



+ Hay, 0. P., Smithson. Misc. Coll., vol. lix, No. 20, p. 3, 1912. 



