Amazingly enough, dinosaurs are divided into two orders:  Saurischia and Ornithischia.  Saurischia (lizard-hipped) are characterized as such because of their forward jutting pubic hipbones and their elongated neck bones.  They are divided into two infraorders:  carnivorous (meat-eating) bipedal (walking on two legs) theropods and herbivorous (plant-eating) quadrupedal (walking on four legs) sauropodomorphs.  Ornithischia (bird-hipped) are characterized as such because their pubis (hipbone) is slanted back parallel to another hipbone and they have a predentary bone at the tip of their toothless lower jaw.  They are plant eaters.  There are three infraorders:  the birdlike ornithpods, the armored thyreophorans, and the horned or thick skulled marginocephalians. 

Marginocephalians belong to the Ornithischia order.  Marginocephalians are characterized as having heads that are formed with a narrow shelf or a deep bony frill. They have broad hips and can either be bipedal (moving on two legs) or quadrupedal (moving on four legs).  They are believed to be herbivores or plant eaters.  Marginocephalians are divided into two infraorders:  Pachycephalosauria and Ceratopsia.

Pachycephalosauria are so named because of their thick skulls.  They range in size from small to moderately large and have broad bodies.  They had small cheek teeth and teeth in front of their jaw.  It is believed they were bipedal plant eaters. Three known families are:

Pachcephalosaurus-which are believed to have had dome shaped heads up to ten inches thick with bony knobs along the base and spikes on its nose.  They are referred to as thickheaded lizards.  They are believed to have been up to 15 feet long with short arms and long legs.

Homalocephalidae-which were flat-headed instead of dome-shaped.

Chaoyangosauridae-which had lobed upper front teeth and canine like teeth.

Certopsia is the second infraorder. They are characterized by their horned faces, a rostral bone that shaped their parrot-like beaks and their flared cheekbones.  It is believed that they used their armor plated skulls and horns to protect themselves.  There is evidence that suggests they lived in herds. The three families are:

Psittacosaurus (parrot lizard)-which is believed to have been 8 feet long.  It walked on two legs and used its tail for balance.  It had a short deep head with ridges of bone on the top.  It did not have horns and frills like true horned dinosaurs.  It is believed that its lifespan was ten to fifteen years.  Fossils were located in China, Thailand and Mongolia.

Protoceratops (first-horned face)- which is believed to have grown up to nine feet long, three feet tall and weighed up to 440 pounds.  It had a large head and broad neck frill. It walked on all four legs.  Fossils were found in Mongolia and China.

Ceratopsids- were known by their armor-plated skulls and horns.  The armor is believed to have been used for protection against predators.  They were plant eaters and ranged in size from a large dog to larger than an elephant.  They lived across the Northern Hemisphere.  There are many well known family members:

Centrosaurus-which has been described as a large rhino.  It is believed to have been very powerful.  It had a large, heavy head with a small head shield.  The frill was not made of solid bone and had two openings.  It had one long, curved horn on his snout and two smaller brow horns.  It had powerful shoulders, thick legs and a small tail.  It grew to be twenty feet in length. 

Triceratops (three-horned face) is best known for its three horns.  It belongs to the ceratopsidae family.  The triceratops is believed to be the largest of its family, weighing up to ten tons.  It had a massive body with a length of up to thirty feet.  Its skull grew up to ten feet long and the skull shield was made of solid bone.  Its bony frill came out from the back of the skull.  Its two brow horns were located above its eyes and were approximately three feet long.  It also had a smaller horn on its nose.

Styracosaurus (spiked lizard) belongs to the ceratopsidae family because of its horned head and neck frill.  One unique characteristic was the six long spikes around the neck frill believed to be used to protect itself.  The nose horn as well as the spikes is believed to be two feet long.  The styracosaurus grew up to three tons and walked on all four legs.  Fossils were found in Alberta, Arizona and Montana.  Fossil evidence suggests they lived in herds.

Microceratops were the smallest of the ceratopsids.  It had long, muscular hind legs, which suggests it was fast.  It had fringe on the back of its skull.  It was believed to be approximately 32 feet long.  Fossils ere located in China and Mongolia.

Pachyrhinosaurus had large, frilled head shield with horns and spikes.  It was twenty-three feet in length and weighed up to three tons.  Fossils were located in Alaska and Alberta.

Anchiceratops was a swamp dweller.  It was twenty feet long and weighed approximately five tons.  It had a long, narrow head shield with spikes pointing backwards.  Fossils were found in Alberta.