Ptilodontoidea is a group of extinct mammals from the northern hemisphere. They were members of an also extinct order called Multituberculata. These were generally small, somewhat rodent-like creatures.
Some of these genera boast a great many species, though remains are generally sparse. Ptilodus is amongst the best known, and there's a tendency to depict it as an analog of a squirrel. Upper Cretaceous remains are known from North America and Europe. (Details about the Europeans would be welcome.) Later representatives, (Paleocene - Eocene), hail from North America, Europe and Asia. These were some of the last multituberculates. For the more technically minded, these animals are within the suborder of Cimolodonta.
The superfamily is further divided into:
Families: Neoplagiaulacidae - 10 genera; Ptilodontidae - 4 genera; Cimolodontidae - possibly 3 genera.
The affinities of Neoliotomus are less clear, though it seems to fit somewhere within the superfamily.
Page reference: Kielan-Jaworowska Z & Hurum JH (2001), Phylogeny and Systematics of multituberculate mammals. Paleontology 44, p.389-429.
(This information has been derived from  MESOZOIC MAMMALS; Ptilodontoidea, an internet directory. As that's my webpage, there are no issues of copyright. Trevor Dykes)