Key to the Skulls of North Dakota Mammals

1. Interparietal fused with parietals (Fig. 15).................................2
   Interparietal distinct, not fused with parietals (Fig. 16)...................3

2. Greatest length of skull more than 85mm.......................Lepus townsendii
                                                         White-tailed Jack Rabbit
   Greatest length of skull less than 85mm.......................Lepus americanus
                                                                    Snowshoe Hare

3. Auditory bullae wrinkled; as long as basioccipital or
     nearly so (Fig. 17).....................................Sylvilagus audubonii
                                                                Desert Cottontail
   Auditory bullae smooth; shorter than basioccipital (Fig. 18).................4

4. Diameter of external auditory meatus more than length of last
     three molariform teeth when measured at the base of
     the crowns (Figs. 19, 20)...............................Sylvilagus nuttallii
                                                             Nuttall's Cottontail
   Diameter of external auditory meatus less than length of last
     three molariform teeth when measured at the base of
     the crowns.............................................Sylvilagus floridanus
                                                               Eastern Cottontail