The Dinosauria superorder was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. The division is based on the bird-like form of the pelvis, the possession of a predentary, details in the vertebrae and armor, and has been generally adopted. The predentary is an extra bone in the front of the lower jaw, and extends the dentary (the main lower jaw bone). The predentary coincides with the premaxilla in the upper jaw. Together they form a beak ...

See also:

Ornithischia, Ornithischia - Characteristics, Ornithischia - Classification, Ornithischia - Taxonomy

Read more here: » Ornithischia: Encyclopedia II - Ornithischia - Characteristics