62 
PALEONTOLOGICAL GEOLOGY 
They are supposed to have been independently developed from 
some didelphid, but the line is continuous back to the Notostylops 
genus Progarzonia, which is just as clearly diprotodont. Then 
there are the other diprotodonts, Palaeothenes, Pilchena, etc. 
Gidley studying the marsupials of the Union beds of North 
America shows that Ptilodus is a marsupial, and that the Allotheres 
and Multituberculates also belong in this group, and although not 
ancestral-are closely related to the diprotodonts. This makes it 
clear that the divisions of the marsulials were established very 
early, and there is no real difficulty in deriving the South American 
forms from early representatives of the group on North America. 
The edentates are already in South America at the time of the 
Notostylops beds. Wortman long ago showed that Psittaco- 
therium, Onchodectes, Conoryctes, etc. were edentates from the 
Puerco and Torreon of North America, so that these seem to come 
from a common source. 
The rodents are perhaps the most puzzling group, for they 
appear suddenly as full flegded rodents in Paramys in the Wasatch 
of North America, and as suddenly in South America in the 
Deseado (Oligocene), but in the latter case they are hystrico- 
morphs. 
Primates are scarce as fossils, but the Puerco of North 
America yields Indrodon, from which, or from related form, the 
Oligocene Clenialites, and the Miocene Homunculus seem to be 
derived, the latter showing already the characteristics of the 
Cebidae. 
Among the herbivors. North America in the oldest Eocene beds 
had several genera, grouped to make the Condylarthra. In like 
manner the Eocene beds of South America have a large variety of 
ungulates which Ameghino placed in this condylarth group cor¬ 
rectly. In North America there rose from this primitive group 
perissodactyls and probably Amblypoda. In South America this 
group gave rise to typotheres, litopternos-and toxodonts. 
There is then, nothing to involve a land-bridge across the oceans; 
on the contrary the early forms in both North and South America 
seem to have been closely related and derived from a common 
source. Review of the reptiles and the fish, especially the Eocene 
ones, shows also a very close relationship between the two con¬ 
tinents. These leave the arguments to those based on land snails 
