1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 529 



Comparison of the Rieardo and Thousand Creek faunas is difficult 

 because of absence of comparable material. The known Carnivora 

 of the Rieardo seem at least as progressive as those of Thousand 

 Creek. In the Equidae the Rieardo Pliohippus species seem near 

 the stage of evolution of the forms from Thousand Creek. The 

 Rieardo hipparions differ generally from what we know of the Thou- 

 sand Creek representatives in less compression of the protocone of 

 the upper cheek-teeth. In this respect the Thousand Creek forms 

 appear more progressive than those of Rieardo. 



Merycoelus, so well represented in the Miocene, is known in the 

 Rieardo but not at Thousand Creek. In the place of Merycoelus, and 

 possibly derived from it, we find in the Thousand Creek several 

 advanced types of antelopes all unknown in the Rieardo. The oreo- 

 dont group represented in the Rieardo is not known to be represented 

 at Thousand Creek. 



As a whole, the Thousand Creek fauna seems more advanced than 

 that of the Rieardo. The difference may be due to geographic vari- 

 ation or to earlier appearance in the Thousand Creek region of 

 immigrants reaching North America in Pliocene time. It is probable 

 that the difference between the Rieardo and Thousand Creek faunas 

 is due in part to difference in stage of evolution, with the Rieardo 

 as the earlier stage. It is not probable that this difference amounts 

 to more than a small portion of a geological period. 



The relationship of the Rieardo and Rattlesnake faunas is pre- 

 sumably similar to that between Rieardo and Thousand Creek, as the 

 Rattlesnake and Thousand Creek faunas seem in general closely 

 related. Such comparison as can be made suggests placing the Rattle- 

 snake somewhat later than the Rieardo and near the position of the 

 Thousand Creek. 



Within the Great Basin province the relationships of the Rieardo 

 fauna so far as known seem somewhat nearer to the Rattlesnake than 

 to any other fauna. The Rieardo represents a stage between the 

 Upper Miocene Barstow and the Lower Pliocene Rattlesnake and 

 Thousand Creek. 



