Introduction 



uncertain just where they branched off from the "family tree" 

 and it is convenient to consider them first, though they are 

 without doubt degenerate animals derived from some ancient ter- 

 restrial forms and are not themselves primitive. The remaining 

 orders fall naturally into two series, those with compressed, 

 hooked "claws" on the feet and those with flat nails or hoofs. 

 We will have then the following table of "orders" of mod- 

 ern mammals: 



Aquatic, with no hind legs and with fore legs modified into 

 flippers for swimming, tail broad and flat; hair little or none. 

 Nostrils opening on top of the head in a "blow hole," 

 teeth, if any, simple and all alike, not tuberculate. 



Cetacea, whales. 



Nostrils at the end of the nose as usual, tuberculate 

 teeth in the back part of the jaws. Sirenia, manatees. 

 Terrestrial (except seals and bats) with all four limbs well devel- 

 oped, and body covered with hair. 

 Nails of feet compressed and hooked forming claws. 

 No incisor teeth; teeth without enamel. 



Edentata, sloths, armadillos, etc. 

 With incisor teeth; enamel present. 



Incisors large and prominent, two in each jaw, concealed 

 portion curved and reaching far back in the skull, canines 

 wanting, leaving a broad gap on each side of the mouth. 



GHres, rats, etc. 



Incisors small, generally more than two, canines present 

 leaving no gap at the side of the jaws. 



Anterior limbs modified into wings ... Chiroptera, bats. 

 Anterior limbs normal. 



Canines not prominent Insectivora, shrews, etc. 



Canines prominent Carnivora, cats, dogs, etc. 



Nails flat or developed into hoofs. 

 Nose modified into a trunk, toes 5. 



Proboscidea, elephants. 

 Nose normal, feet never 5-toed, always armed with hoofs. 



Ungnlata, horses, cows, etc. 

 Nose normal, feet always 5-toed. 



Primates, monkeys and man, 



There are a few more or less obscure foreign mammals that 

 are not accommodated in the scheme given above, and which are 

 intermediate in their characters. 



In North America we lack representatives of several orders. 

 The Prototheria are entirely wanting and of the Marsupialia we 



xvn 



