You don't say where you found these but assuming they are from Britain it looks like part of the right lower jaw of a mouse. The possibilities are Wood or Yellow necked mouse, they have very similar teeth. House mouse has a different pattern to the front lower molar and so I don't think this is a house mouse. Wood mouse are found throughout britain but the Yellow necked mouse is much more restricted. If you post where it was found then it may help to exclude yellow necked mouse.
In the most recent Mammal society survery the yellow necked mouse was found in Southern Britain and up the severn valley as far as Stafford, so no where near Lincolnshire. So this is almost cetainly from a wood mouse
YNM were recorded in Liconlshire in the late 1950's (not sure exactly where). I recently found them at a site in Bedfordshire from which they have not been recorded since 1960 so don't assume a lack of recent records means a lack of the species being present. You will have to content your self with "Apodemus sp." as the identification for this jaw fragment.