Cimexomys is an extinct, North American mammal that lived during the Upper Cretaceous - Paleocene. For a while, it shared the world with dinosaurs, but outsurvived them. It was a member of the also extinct order of Multituberculata.

For those of a technical disposition, it's within the Suborder of Cimolodonta, and perhaps a member of the Paracimexomys group, though that's not certain. Most remains are restricted to teeth.

Genus: Cimexomys Sloan & Van Valen, 1965
'bug mouse'
Aka: Eucosmodon sp., Mesodma?, Stygimys sp. ('Styx mouse')
Remarks: "Looking into the original description reveals that "Cimex" means "bug", with reference to the type locality of Cimexomys minor, the Bug Creek Anthills in Montana, USA. Next question would be why that place is called Bug Creek..." (Many thanks to Martin Jehle for the information. Maybe the river's got six legs, but I doubt it.)
The inclusion of C. in the Paracimexomys group is tentative, (Kielan-Jaworowska & Hurum, 2001, p.403).
Some species are termed here as "Cimexomys", and they're listed towards the end. New species have previously been christened with great enthusiasm, and much reidentification has also occurred.

Species: Cimexomys arapahoensis (Unpublished)
Place: Denver Formation, Colorado
Country: USA
Age: Lower Paleocene
Remarks: "C. arapohoensis is known from four dental and mandiblar fossils from the Alexander Locality of the Denver Formation (Colorado) and is the largest known species of Cimexomyx. I estimated its mass at about 120 g based on its lower first molar." (Eric W Dewar (2002), and many thanks for the information! Pers communication.) C. minor has also been identified from this location.
Reference: Not as yet.
Species: ? Cimexomys gratus (Jepsen GL, 1930) Lofgren DL 1995
Aka: Cimexomys hausoi Archibald, 1982; Eucosmodon gratus Jepsen, 1930; Mesodma? ambigua? Jepsen, 1940, Stygimys gratus (Jepsen, 1930) Place: Polecat Bench Formation, Wyoming
Country: USA
Age: Puercan, Paleocene
Remarks: Confusing. I wonder if this will turn out to be synonymous with Stygimys kuszmauli, which also lives in the Peabody Museum, Yale. The animal weighed something like 130g, (five mice or a third of a rat).
Reference:
Species: Cimexomys judithae Sahni A, 1972
Aka: Paracymexomys judithae, Archibald JD, 1982
Place: Two Medicine Formation, Montana
Country: USA
Age: Campanian, Upper Cretaceous
Remarks: A near complete dentition and skeletal elements were discovered at Egg Mountain, a site associated with a dinosaur nesting colony, though this wasn’t the original material. C. judithae is reportedly more derived than other species, and the integrity of the genus is suspect, (as reflected here by the later entries for "Cimexomys"). Montellano reassigned this species to the cimex genus in 1992. Estimated body mass = 20g.
Based on the new material from Montana, the authors "argued that Cimexomys is too primitive to be included among either the Taeniolabidoidea or Ptilodontoidea. This agrees with our conclusion as we tentatively place Cimexomys in the informal 'Paracimexomys group', which we erect for plesiomorphic, poorly known members of the Cimolodonta, (Kielan-Jaworowska & Hurum, 2001, p.393). Plesiomorphic means basal.
Reference: Sahni (1972), Bulletin of the American Museum of Natural History 147 (6).
Species: Cimexomys minor Sloan RE & Van Valen L, 1965
Place: Hell Creek Formation, Montana & Denver Formation, Colorado & Ravenscrag Formation, Saskatchewan
Country: USA & Canada
Age: Upper Cretaceous - Lower Paleocene
Remarks: Small teeth of an eighth to a tenth of an inch in length. The animal weighed about the same as a standard mouse, (25g).
Reference: Sloan & Van Valen (1965), Cretaceous mammals from Montana. Science 148, p.220_227.
Species: "Cimexomys" antiquus Fox RC, 1971
Place: Utah
Country: USA
Age: Santonian, Upper Cretaceous
Remarks: The type fossil is in Alberta. A 30g multi.
Reference: Fox (1971), Canadian J of Earth Sci 8, p.916.
Species: "Cimexomys" gregoryi Eaton JG, 1993
Place:
Country: USA
Age: Campanian, Upper Cretaceous
Remarks: Shows similarities to Bryceomys.
Reference:
Further names: Information welcome!
"Cimexomys" bellus, "C." clarki, "C." electus,

Page references: Kielan-Jaworowska Z & Hurum JH (2001), Phylogeny and Systematics of multituberculate mammals. Paleontology 44, p.389-429.

(This information has been derived from [1] (http://home.arcor.de/ktdykes/cimolod.htm) MESOZOIC MAMMALS; 'basal' Cimolodonta, Cimolomyidae, Boffidae and Kogaionidae, an internet directory. As that's my webpage, there are no issues of copyright. Trevor Dykes)