254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
J 
eke) 
See 
He 50 fe) 
million years 
CRETACEOUS PALAEOGENE NEOGENE 
FIGURE 2,—Diagram of the numbers of species in the sea-urchin genus Salenia 
from the Cretaceous to the present day. (Surface area proportional to the 
number of species.) 
TATA 200 Tele) i 
400 
million years 
U.CAMB.ORD. SILUR.DEV. CARB. PERM, TRIAS.JURAS.CRET. CAINOZOIC 
Ficure 3.—Number of species in the brachipod genus Lingula, from the Upper 
Cambrian to the present day. (Surface area proportional to the number of 
species.) 
