Key to the Skulls of North Dakota Mammals

1. Upper incisors absent (Fig. 1)....................................ARTIODACTYLA
   Upper incisors present (Fig. 2)..............................................2

2. Five upper incisors on each side.....................MARSUPIALIA (Didelphidae)
                                                             Didelphis virginiana
                                                                 Virginia Opossum
   Less than five upper incisors on each side...................................3

3. Diastema between incisors and molariform teeth, canine absent (Fig. 3).......4
   Diastema absent..............................................................5

4. Two pairs of upper incisors (second pair behind first pair and 
     much smaller)(Fig. 4).................................LAGOMORPHA (Leporidae)
   One pair of upper incisors............................................RODENTIA

5. Upper incisors of right and left sides separated by a space (a deep anterior
     emargination of the palate)(Fig. 5)............CHIROPTERA (Vespertilionidae)
   Upper incisors of both sides in a continuous row (no space)(Fig. 6)..........6

6. Canines prominent, longer than premolars or incisors (Fig. 2)........CARNIVORA
   Canines not well developed, seldom longer than premolars or
     incisors (Fig. 7)....................................INSECTIVORA (Soricidae)