None of these have the bony frill completely roofing over the neck as it does in _Triceratops_.
There is always a central spine projecting backwards and widening out at the top to the bony margin of the frill which sweeps around on each side to join bony plates that project from the sides of the skull top.
This encloses an open space or "fenestra," so that the neck was not completely protected above.

Sometimes the margin of the frill is plain, at other times it carries a number of great spikes, like a gigantic Horned Lizard (_Phrynosoma_).
[Illustration: Fig.

40 .-- Outline sketch restoration of _Triceratops_, from the mounted skeleton in the National Museum.] In _Ceratops_ the horns over the eyes are large and the nasal horn small.

In _Monoclonius_ the nasal horn is large and those over the eyes are rudimentary.

The great variety of species that has been found in recent years shows that these Horned Dinosaurs were a numerous and varied race of which as yet we know only a few.