Silurians are scaly, cold-blooded reptilian humanoids, with yellow eyes and oval mouths. They are most probably descendants of the hadrosaurs.
There were at least to subspecies of SIlurians. An amphibious one, and a landbase one. Some specimens of the latter possessed a third eye with various abilities, such as defense, wether it was stunning or killing, communication or activating mechanisms keyed to emanations from the eye. The eye usually glowed red when active.
They lived tens of millions of years in the past. Some of them went into suspended animation when faced with some plantewide cataclysm. They were scientifically advanced with expertise in genetic engineering.
One of those group were awakened in the late 1960ies by the energy from a nearby nuclear power research centre. The Silurians then attempted to reclaim the planet from humanity by releasing a deadly virus and attempting to disperse part of the Van Allen Belt. Both plans, however, were thwarted. The Silurian base was destroyed by UNIT.
Note: The Silurians resemble, both conceptually and visually, the Voth race from the episode “Distant Origin” of Star Trek Voyager, in which the Voth, like the Silurians, are an advanced reptile species who evolve on Earth before Humanity and subsequently disappear. There is also a strong physical resemblance between the costumes and make up used for the two alien races.Source:
Doctor Who “Doctor Who and the Silurians”