0. C. Marsh— Vertebrate Life in America. 353 
important to define the division Lpstiy me indicated in our 
Tertiary and Post-Tertiary aapoihe as these in many cases 
eae series of the West is uppermost Cretaceous, or lowes 
Kocene. The evidenoe of the numerous vertebrate remains is 
are most setae witnesses ; that invertebrate animals 
are much better; and that vertebrates afford the most reliable 
evidence of climatic and other geological changes. e su 
divisions of the latter group, moreover, and in fact all forms 
of animal life, are of value in this respect, mainly according 
to the gf molgese4 of their organization, or Zoologica al rank. 
) 
law, ene that the line, it sc ae a separating our 
Cretaceous from the Tertiary, m t at present be drawn where 
the Dinosaurs and other M sete vertebrates disappear, and 
are replaced by the Mammals, nencetirts the dominant type. 
The Tertiary of Western n America ca comprises the most exten- 
sive series of. deposits of this age known to geologists, and 
important breaks in both the rocks and the fossils separate it 
into three well-marked divisions. These natural divisions are 
not the exact equivalents of the Kocene, Goons and Pliocene 
Am. Jour. Sct. ime are Vou. XIV, No. 83.—Noy., 1877, 
