(_Microtus_) _mexicanus_ (Saussure) 13 (_Microtus_) _californicus_ (Peale) 2 (_Microtus_) _pennsylvanicus_ (Ord) 13 (_Microtus_) _agrestis_ (Linnaeus) 1 (_Pedomys_) _ochrogaster_ (Wagner) 41 (_Pitymys_) _pinetorum_ (LeConte) 2 (_Pitymys_) _parvulus_ (Howell) 1 (_Pitymys_) _quasiater_ (Coues) 5 (_Pitymys_) _fatioi_ Mottaz 1 Genus: _Neofiber_ True, 1884 _Neofiber alleni_ True 2 Genus: _Lagurus_ Gloger, 1841 _Lagurus curtatus_ (Cope) 7 Total number examined 184 ACCOUNTS OF SPECIES Dicrostonyx groenlandicus (Traill) Figs. 3 and 4 Baculum: stalk elongate, greatest length (3.1 mm.) 2-1/5 to 2-1/2 times greatest breadth, and 4-1/2 times greatest depth; digitate processes usually cartilaginous, occasionally lateral processes partly ossified; basal tuberosities weakly to moderately developed, medially confluent; posterior profile in dorsal view rounded with rounded posterior apex or shallow notch; dorsal concavity in end-view shallower and not so wide as ventral concavity; median constriction approximately 2/3 greatest depth; ventral part of base in end-view wider than dorsal part; shaft straight or slightly curved; base of stalk placed dorsally relative to axis of shaft; stalk spatulate, sometimes with distal enlargement; at mid-point stalk wider than high; lateral profile in dorsal view sloping gradually without abrupt curvature anterior to point of greatest width. The baculum of _Dicrostonyx torquatus_ figured by Ognev (1948:476) agrees with that of _D. groenlandicus_ in shape of stalk, and in lateral digitate processes that are small relative to size of median process; but differs in more elongate, terminally enlarged, bulbar shape of median process. None of my specimens showed ossification in the lateral processes, observed by Hamilton (1946:381) in _Dicrostonyx rubricatus richardsoni_ [ = _D. groenlandicus richardsoni_]. In all of my specimens the cartilaginous median process was larger than that figured by Hamilton, or by Dearden (1958:542). _Specimens examined_: Four from; Point Barrow, Alaska, 52524 (Barrow Village), 67264 (died in captivity); Brooks Range, Alaska, 50536 (Wahoo Lake, 69°08', 146°58'), 50539 (Porcupine Lake, 68°51'57", 146°29'50", 3140 ft.). Lemmus trimucronatus (Richardson) Fig. 2