100 University of California Publications in Geology [VoU-.7 
The earliest occurrence of rheids 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** 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. Lueas,*° 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 
Jocal 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 bionomie place lke that of the mammalian 
sarnivore. 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 
34 Cope, E. D., U. S. Geol. Surv. Terr. W. of 100th Merid., vol. 4, pt. 2, 
p. 69, 1876. 
35 Lueas, F. A., Proce. U. S. Nat. Mus., vol. 24, p. 545, 1903. 
