Troodontidae

Troodontidae is a family of bird-like theropod dinosaurs. They were found mainly in Mongolia, China and North America. The first specimen, just a single tooth, was found in central Montana in 1855 by Ferdinand Hayden.
For a long time, troodontid fossils were few and scrappy. Recently more complete specimens have been found. There are now complete and articulated specimens, including feathers, eggs and embryos, and complete juveniles. In 1994 a fossil of an adult "Troodon" was found sitting on a nest with eight eggs.
Anatomical studies of the most primitive troodontids, like "Sinovenator", demonstrate striking similarities with "Archaeopteryx" and primitive dromaeosaurids. This proves they are relatives: they are a clade called Paraves. 
The family have relatively large brains, a skull with many air pockets, a retractable second toe claw (it could be pulled in) and many teeth. The number of teeth ranges from 96 in "Saurornithoides mongoliensis" to 120 in the "Troodon".