100 University of California Publications in Geology [V 0L - 7 



The earliest occurrence of rheicls in South America is in 

 strata now referred to the Pleistocene (the Pampean of Monte 

 Hermosa). If the group had reached that continent by way 

 of the Antarctic at an earlier time, their bones would probably 

 be found with the primitive mammals supposed to have been 

 derived from Australia and known to us from the Santa Cruz 

 beds. The rheas with their true struthious characters could 

 hardly have originated de novo in South America; hence the 

 conclusion that they entered from the north, as did the true 

 cats, deer, elephants and other mammals of northern or Old 

 World origin. 



Cope's discovery of Diatryma 34 in the Wahsatch Eocene of 

 New Mexico was at first considered as fixing a very early date 

 for the group of Struthiones in the New World. Lucas, 35 how- 

 ever, places this unique specimen in the group of Stereornithes 

 with the great Phororhacos of South America (Miocene of Santa 

 Cruz). A wide gulf exists between the ostriches and these 

 South American phororhacids. The latter are more probably a 

 local development brought out in response to the peculiar con- 

 ditions prevailing there in Tertiary time. There existed in South 

 America no large carnivores among mammals until the northern 

 incursion of machaerodonts and the true felines in relatively late 

 geological time. Edentates were left free to develop to the 

 tremendous extent noticeable in the South American Tertiary 

 and Quaternary. In this region of low pressure among mam- 

 mals there developed unrestrained the predatory bird Phoro- 

 rhacos, to occupy a bionomic place like that of the mammalian 

 carnivore. The reference by Lucas of the North American Dia- 

 tryma to the Stereornithes is tentative. He states the case in 

 these words in part : ' ' Still there are sufficient resemblances be- 

 tween the two to warrant the suggestion that if material comes 

 to light it will be found that the affinities of Diatryma are with 

 the Stereornithes and not with the Dromaeognathae. " 



In view of the indeterminate character of the single specimen 

 of Diatryma where its relationship between two such distinct 



S4 0ope. E. D., U. S. Geo!. Surv. Terr. W. of 100th Merid., vol. 4, pt. 2, 

 p. 69, 1876. 



33 Lucas, F. A., Proc. TJ. S. Nat. Mus., vol. 24, p. 545, 1903. 



