e best known American genus.

2. _Trachodonts_ or _Duck-billed Dinosaurs_. Like the Iguanodonts but
with numerous rows of small teeth set close together to form a
grinding surface. Cretacic period. _Trachodon, Hadrosaurus,
Claosaurus, Saurolophus, Corythosaurus, etc._

3. _Stegosaurs_ or _Armored Dinosaurs_. Quadrupedal dinosaurs with
elephantine feet, short neck, small head, body and tail armored with
massive bony plates and often with large bony spines. Teeth in a
single row, like those of Iguanodonts. _Stegosaurus_ of the Upper
Jurassic, _Ankylosaurus_ of the Upper Cretacic.

[Illustration: Fig. 8.--Hind Feet of Dinosaurs, to show the three
chief types (Theropoda, Orthopoda, Sauropoda).]

4. _Ceratopsian_ or _Horned Dinosaurs_. Quadrupedal with elephantine
feet, short neck, very large head enlarged by an enormous bony frill
covering the neck, with a pair of horns over the eyes and a single
horn in front. Teeth in a single row, but broadened out and adapted
for grinding the food. No body armor. _Triceratops_ is the best known
type. _Monoclonius_, _Ceratops_, _Torosaurus_ and _Anchiceratops_ are
also of this group. All from the Cretacic period.

_Classification of Dinosaurs._ It is probable that the Dinosaurs are
not really a natural group or order of reptiles, although they have
been generally so considered. The Carnivorous and Amphibious Dinosaurs
in spite of their diverse appearance and habits, are rather nearly
related, while the Beaked Dinosaurs form a group apart, and may be
descendants of a different group of primitive reptiles. These
relations are most clearly seen in the construction of the pelvis (see
fig. 9). In the first two groups the pubis projects downward and
forward as it does in the majority of reptiles, and the ilium is a
high rounded plate; while in the others the pelvis is of a wholly
different type, strongly suggesting the pelvis of birds.

[Illustration: Fig. 9.--Pelves of Dinosaurs illustrating the two
chief types (Saurischia, Ornithischia) and their variations.]

Recent researches upon Triassic dinosaurs, especially by the
distinguished German savants, Friedrich von Huene, Otto Jaekel and the
late Eberhard Fraas, and the discovery of more complete specimens of
these animals, also clear up the true relationships of these
primitive dinosaurs which have mostly been referred hitherto to the
Theropoda or Megalosaurians. The following classification is somewhat
more conservative than