; five
cheek teeth on each side above
Family Ochotonidae, Genus _Ochotona_, p. 125

1'. Hind legs notably larger than forelegs; hind foot more than 40;
nasals widest posteriorly; supraorbital process on frontal; six
cheek teeth on each side above
Family Leporidae, p. 134

2. Interparietal fused with parietals (see fig. 49); hind foot
usually more than 105
Genus _Lepus_, p. 170

2'. Interparietal not fused with parietals (see fig. 10); hind foot
usually less than 105
Genera _Romerolagus_ and _Sylvilagus_, pp. 137, 138

Family OCHOTONIDAE--Pikas

Certain characters in which this family differs from the Leporidae
(hares and rabbits) are: hind legs scarcely longer than forelegs; ears
short, approximately as wide as high; no postorbital process on frontal;
rostrum slender; nasals widest anteriorly; maxilla not conspicuously
fenestrated; jugal long and projecting far posteriorly to zygomatic arm
of squamosal; no pubic symphysis; one less cheek-tooth above, the dental
formula being i. 2/1, c. 0/0, p. 3/2, m. 2/3; second upper maxillary
tooth unlike third in form; last lower molar simple (not double) or
absent (in the extinct genus _Oreolagus_); cutting edge of first upper
incisor V-shaped; mental foramen situated under last lower molar.

Genus OCHOTONA Link--Pikas

Revised by A. H. Howell, N. Amer. Fauna, 47:1-57, August 21, 1924.

1795. _Ochotona_ Link, Beytr[:a]ge zur Naturgesch, I (pt. 2):74. Type,
_Lepus ogotona_ Pallas.

_Characters_.--Five teeth (excluding incisor) in lower jaw; first
cheek-tooth (p3) with more than one re-entrant angle; columns of lower
molars angular internally; transverse width of any one column of a
lower molariform tooth more than double the width of the neck connecting
it to the other column.

Subgenus PIKA Lac['e]p[e']de

1799. _Pika_ Lac['e]p[e']de, Tableau des Divisions &c., Mamm., p. 9.
Type, _Lepus alpinus_ Pallas.

1904. _Pika_, Lyon, Smiths. Misc. Coll., 45:438, June 15.

_Characters._--Skull flattened; interorbital region wide; maxillary
orifice roundly triangular; palatal foramina separate from anterior
palatine foramina.

All of the living members of the family Ochotonidae belong to this
genus. American pikas all belong to the subge