1 a.

Teeth or tooth sockets presen; skull of relatively thick bone...................................................(mammals)...2

b.

Lacking teeth or tooth sockets; skull or relatively thin bone...........................................................(birds) Aves

2 a.

A wide gap on each side between front teeth and chewing teeth..................................................................3

b.

Tooth row continuous (no wide gap)............................................................................(Order Insectivora)...6

3 a.

With only one front tooth on each side having its surface stained
tan or reddish orange.........................................................................................................(Order Rodentia)......4

b.

With two upper front teeth on each side, the second being smaller
than the first, no coloration on the front surface teeth............................(Rabbits, Order Lagomorpha)

4 a.

Skull total length less than 32 mm; rows of cheek teeth separated in
front less than 3mm ...............................................................................................................(Mice or Voles)....5

b.

Skull over 32mm in length; cheek teeth rows separated by over 3.5 mm ..............(Rats, Genus Ratus)

5 a.

Cheek teeth rows over 5 mm in length, chewing surface worn smooth
and flat.......................................................................................................................(Vole, Genus Microtus)

b.

Cheek teeth rows less than 5 mm in length, chewing surface with "cones" or
"cusps" (not flat)..........................................................................................................(Mice, Genus Microtus)

6 a.

Tips of teeth reddish..................................................................................................(Shrews, Insectiviroa)

b.

Tips of teeth all white or with grayish stain..............................................................(Moles, Insectivora)