DIVISIONS OF ZOOLOGY. 15 
unequal, and the most probable estimate of the comparative 
duration of each is obtained by measuring the thickness of 
the rocks belonging to each. The following table represents 
approximately the relative duration of each period, and 
opposite them are the names of the different sub-kingdoms 
and classes of animals, which, as far as our knowledge goes, 
then made their first appearance :— 
Pliocene ......00- : 
CAINOZOIC < Miocene.......... HH 
LO ee | EO Ae PE re Placental Mammalia 
Cretaceous....... 
WERBOZORC he ABRs ke cece FEL ves cecedigacccsuepbatets Aves 
TESST Al hiacates Fed cacao Senne caeeeene kal Marsupial Mammalia 
EPPO rn AS Reptilia 
Carboniferous Ry cacuctonsesecuenesnenaxs Amphibia 
oth itekweaaip onan hese, Insecta 
PAL#ozoIc4 Devonian....... 
Vertebrata ......... Pisces 
Upper Silurian 
Meise cdadvcnnvanaany sans Gasteropoda 
| Lower Silurian ZV ....scsseeeseeseeeeeees Lamellibranchiata 
EP TT eh eee e Polyzoa 
Tr eer rree er Actinozoa 
4 | Mollusca....cesa++s- Pteropoda 
Arthropoda ........ Cephalopoda; Crustacea 
Cambri Kchinodermata...| Brachiopoda 
gor lida = oi Sai OV OEIIES Gos sates anes Porifera 
EOZOIC...... { Celenterata........|Hydrozoa 
Pratozowd... Secaises Rhizopoda 
( Laurentian..... 
Another result of the study of Paleontology, is to show 
that the present geographical distribution of animals dates 
