Key to the Skulls of North Dakota Mammals

1. Infraorbital opening larger than foramen
     magnum (Fig. 21)..........................Erethizontidae, Erethizon dorsatum
                                                                        Porcupine
   Infraorbital opening smaller than foramen magnum.............................2

2. Infraorbital opening small, fairly inconspicuous; four lower
     molariform teeth on each side..............................................3
   Infraorbital opening of moderate size, vertically elongate; three
     lower molariform teeth on each side (Fig. 22).............................20

3. Postorbital process prominent (Fig. 25)..........................Sciuridae...5
   Postorbital process absent...................................................4

4. Bones thin and translucent or nearly so......................Heteromyidae...17
   Skull not as above..........................................................15

5. Infraorbital opening a foramen piercing the zygomatic
     plate (Fig. 23)............................................................6
   Infraorbital opening a canal passing between the zygomatic plate
     and the rostrum............................................................7

6. One upper premolar.............................................Tamias striatus
                                                                 Eastern Chipmunk
   Two upper premolars...........................................Eutamias minimus
                                                                   Least Chipmunk

7. Postorbital process very broad, projecting at nearly right angles to long
     axis of skull at about the middle of the orbit (Fig. 24).......Marmota monax
                                                                        Woodchuck
   Postorbital process projecting posteriorly and ventrally (Fig. 25)...........8

8. Zygomatic arches nearly parallel, not twisted (Fig. 26)......................9
   Zygomatic arches converging anteriorly; ventro-medial portion
     twisted toward a horizontal plane (Fig. 27)...............................12

9. Five upper molariform teeth, the first reduced in size......................10
   Four upper molariform teeth, all about the same size........................11

10. Greatest length of skull less than 50mm; interorbital region
      narrow, deeply notched (Fig. 28).........................Glaucomys sabrinus
                                                         Northern Flying Squirrel
    Greatest length of skull more than 50mm; interorbital region relatively
      broad, not deeply notched (Fig. 29)....................Sciurus carolinensis
                                                                    Gray Squirrel

11. Zygomatic breadth usually less than 30mm; antero-ventral border
      of orbit opposite first upper molariform
      tooth (Fig. 30).....................................Tamiasciurus hudsonicus
                                                                     Red Squirrel
    Zygomatic breadth usually more than 30mm; antero-ventral border of
      orbit opposite second upper molariform tooth (Fig. 31)........Sciurus niger
                                                                     Fox Squirrel

12. Upper molar rows strongly converging posteriorly.........Cynomys ludovicianus
                                                         Black-tailed Prairie Dog
    Upper molar rows nearly parallel...........................................13

13. Greatest length of skull more than 50mm...............Spermophilus franklinii
                                                       Franklin's Ground Squirrel
    Greatest length of skull less than 50mm....................................14

14. Zygomatic breadth less than 25mm................Spermophilus tridecemlineatus
                                                   Thirteen-lined Ground Squirrel
    Zygomatic breadth more than 25mm....................Spermophilus richardsonii
                                                     Richardson's Ground Squirrel

15. Basioccipital concave (cup-like) with lateral edges projecting
      ventrally.....................................Castoridae, Castor canadensis
                                                                           Beaver
    Basioccipital with elevated median ridge, or
      not as above.................................................Geomyidae...16

16. Upper incisors with prominent grooves in
      front (Fig. 32)............................................Geomys bursarius
                                                             Plains Pocket Gopher
    Upper incisors not grooved, or only
      faintly so (Fig. 33).....................................Thomomys talpoides
                                                           Northern Pocket Gopher

17. Mastoid area greatly expanded; interparietal width less than 1/4
      of greatest skull width (Fig. 34)...........................Dipodomys ordii
                                                               Ord's Kangaroo Rat
    Mastoid area moderately or slightly expanded; interparietal width
      more than 1/4 of greatest skull width (Fig. 35)..........................18

18. Breadth of interparietal equal to or greater than interorbital
      breadth; auditory bullae separated by almost entire
      width of basisphenoid (Fig. 36)........................Perognathus hispidus
                                                              Hispid Pocket Mouse
    Breadth of interparietal less than interorbital breadth;
      auditory bullae meeting or nearly so anteriorly (Fig. 37)................19

19. Auditory bullae meeting anteromedially (Fig. 38).......Perognathus flavescens
                                                              Plains Pocket Mouse
    Auditory bullae not meeting (Fig. 37)...................Perognathus fasciatus
                                                        Olive-backed Pocket Mouse

20. Three upper molars and one small upper premolar; infraorbital
      canal oval...................................................Zapodidae...21

    Three upper molars and zero upper premolars; infraorbital
      canal "V" shaped.........................................................22

21. Palatal breadth at third upper molar less than 4.2mm; incisive
      foramen less than 4.6mm (Fig. 39)...........................Zapus hudsonius
                                                             Meadow Jumping Mouse
    Palatal breadth at third upper molar more than 4.4mm; incisive
      foramen more than 4.6mm......................................Zapus princeps
                                                            Western Jumping Mouse

22. Upper molars with three longitudinal rows of cusps...............Muridae...23
    Upper molars with two longitudinal rows of cusps
      (or without cusps)..........................................Cricetidae...24

23. Greatest length of skull more than 25mm; zygomatic breadth
      more than 15mm............................................Rattus norvegicus
                                                                       Norway Rat
    Greatest length of skull less than 25mm; zygomatic breadth
      less than 25mm.................................................Mus musculus
                                                                      House Mouse

24. Basal length of skull more than 35mm (Fig. 40).............................25
    Basal length of skull less than 35mm.......................................26

25. Basal length of skull more than 50mm.......................Ondatra zebethicus
                                                                          Muskrat
    Basal length of skull less than 50mm..........................Neotoma cinerea
                                                             Bushy-tailed Woodrat

26. Occlusal surface of cheekteeth lacking cusps, but with
      dentine lakes............................................................27
    Occlusal surface of cheekteeth with cusps..................................30

27. Auditory bullae and mastoids much enlarged, reaching plane
      of occipital condyles (Fig. 41)............................Lagurus curtatus
                                                                   Sagebrush Vole
    Auditory bullae and mastoids not greatly enlarged..........................28

28. Palate ending in straight shelf (Fig. 39)...............Clethrionomys gapperi
                                                         Southern Red-backed Vole
    Palate not ending in straight shelf........................................29

29. Second upper molar with four closed angular sections and a
      rounded posterior loop; third upper molar with five
      dentine lakes (Fig. 42).............................Microtus pennsylvanicus
                                                                      Meadow Vole
    Second upper molar with four angular sections and no posterior
      loop; third upper molar with four dentine
      lakes (Fig. 43)........................................Microtus ochrogaster
                                                                     Prairie Vole

30. Coronoid process of lower jaw enlongated, well
      developed (Fig. 44)...................................Onychomys leucogaster
                                                       Northern Grasshopper Mouse
    Coronoid process of lower jaw short, reduced (Fig. 45).....................31

31. Upper incisors with a prominent groove.....................................32
    Upper incisors without groove..............................................33

32. Breadth of braincase 9.5mm or more; small accessory cusp
      between first and second larger cusps evident on outer surface
      of first lower molar (Fig. 46)....................Reithrodontomys megalotis
                                                            Western Harvest Mouse

    Breadth of braincase not more than 9.6mm; no accessory cusp
      evident............................................Reithrodontomys montanus
                                                             Plains Harvest Mouse

33. Peromyscus leucopus and Peromyscus maniculatus (see Note on Peromyscus
      at end of key).............................................................