830 SLODIES FORZS LODENTS 
characters of the recent forms. The first group, the Archaeocet, 
is represented by a form that is known from all parts of the 
world, Zeuglodon; it has an elongate skull like the alligator, but 
still possesses the dentition of the land animals, in that it is dif- 
ferentiated into premolars and molars; the position of the nos- 
trils and the extent of the nasal bones are also typical of the land 
forms, but the itmbs are those of a water animal. As the form 
is the earliest known, it is regarded as the nearest to the primi- 
tive ancestor of the Cefacza. The carnivorous dentition has led 
to the conclusion that it, and consequently all of the order was 
derived from a carnivorous mammal. The high degree of devel- 
opment of the order at its earliest appearance, indicates that 
these animals must have begun their specialization some time in 
the Cretaceous before we know of any mammals that could have 
produced them. 
The Szrenta have a like history to the foregoing group. 
They are, without doubt, the descendants of land-living forms, 
but are derived from ungulates, probably from the primitive 
Condylarthra, instead of from carnivorous forms. The earliest 
remains are from the Eocene-rocks, and show that the animal 
was at that time still in possession of a pair of rudimentary hind 
limbs. Specimens are known from most of the countries of the 
earth, and from all deposits from the earliest Eocene up. 
Riya, steller's Sea- cow, became extinck as latemasmrle 
middle of the eighteenth century. It was found in great 
abundance on the shores of Alaska and the neighboring islands 
by the early explorers, and was slaughtered for food by the 
whalers. 
Ungulata.—This is one of the largest groups of the mam- 
mals, including all of the herbivorous forms with the exception 
of the rodents. They are all land-living forms, with the limbs 
modified as organs of locomotion and the terminal phalanges 
armed with broad, flat, horny coverings or hoofs. The dentition 
is adapted to a vegetable diet or to an omnivorous one, as in the 
pig. The dentition is diphydont,z.¢., there is a milk set that is 
later replaced by a permanent one. 
