1. Length of skull less than 20 mm: Mus musculus.
1'. Length of skull more than 20 mm: Rattus, 2
2. Temporal ridges more or less straight and parallel to one another as they cross the parietal bone; tail length usually less than length of head and body: Rattus norvegicus.
2'. Temporal ridges bowed laterally and diverging posteriorly as they cross the parietal bone; tail length as long as or longer than length of head and body: Rattus rattus.
Key modified from Findley (1987).
Last Update: 2 Nov 2007