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ELEMENTS OF MAMMALIAN ANATOMY. 
the old world, while the opossum is confined to the new 
world. Again, many species of animals whose fossil re- 
mains indicate their existence on earth several millions of 
years ago, have at present no living representatives. A 
consideration of this geographical and stratigraphical loca- 
tion of organisms forms the science of Distribution. The 
science of Phylogeny seeks to discover the geological 
ancestral history of an organism. 
A casual glance shows at once striking similarities and 
differences between the common cat, the lion, and the tiger. 
All have retractile claws, the same number and kind of 
teeth, and the same number of toes. On the other hand, 
the resemblances between these cat-like animals and the 
dogs are less marked, while the differences are more strik- 
ing. The cats and dogs resemble each other more closely 
than either does a horse. Horses, dogs, and cats have 
numerous characters in common which are not present in 
birds. The recognition of such resemblances and differ- 
ences furnishes a basis of classification, the treatment of 
which forms the science of Taronomy. 
The following is a brief classification of the animal 
kingdom: 
Invertebrata: Animals with no skeletal axis and without a central 
nervous system entirely dorsal of the alimentary canal. 
Protochordata: Small marine forms having, during part of their life 
at least, a rudimentary skeletal axis and other fea- 
tures marking them as a connecting-link between the 
invertebrates and vertebrates. Tunicates or sea 
squirts, Amphioxus. 
Vertebrata: Forms with a skeletal axis and dorsal nerve-cord 
whose anterior end is dilated into a brain. 
Pisces (fish). 
Ampuisia (frogs, toads, and salamanders). 
Reetitia (snakes and lizards). 
Aves (birds). 
