The number of molts in a year, depending on the kind of lagomorph, varies in adults from one (according to Nelson, 1909:31) in the cottontails (genus _Sylvilagus_) to as many as three (according to Lyman, 1943, and Severaid, 1945) in the varying hare (_Lepus americanus_). Difficulties that I have experienced in attempting to account for the variations in color and wear of the pelage of the pika, _Ochotona princeps_, on the basis of two molts per year, make me wonder if it, too, has three molts. _Lepus townsendii_ certainly has at least two molts per year.
1. Hind legs scarcely larger than forelegs; hind foot less than 40; nasals widest anteriorly; no supraorbital process on frontal; 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
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.
Revised by A. H. Howell, N. Amer. Fauna, 47:1-57, August 21, 1924.
1795. _Ochotona_ Link, Beyträge 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.