. Bulletin. Science; Natural history; Natural history. Fig. 3. First lower molars of Sigmodon lindsayi, new species, and S. mascotensis. S. lindsayi: A, LACM 124161, holotype LM,, x ; B, LACM XIAXIA, unworn, possibly embryonic LM,, x ; C, LACM 124111, RM, with slightly eroded anteroconid-notice separation of metaconid from an- teroconid, ; D, LACM 124117 RM„ ; E, LACM 124123 LM„ heavily worn, S. mascotensis: F, MSU 12531, LM,, from a female, collected 6 mi. W Autlan, 4400 ft, Jahsco, Mexico, LRA2 is more perpendicular to the midline of the tooth and the tip of t
. Bulletin. Science; Natural history; Natural history. Fig. 3. First lower molars of Sigmodon lindsayi, new species, and S. mascotensis. S. lindsayi: A, LACM 124161, holotype LM,, x ; B, LACM XIAXIA, unworn, possibly embryonic LM,, x ; C, LACM 124111, RM, with slightly eroded anteroconid-notice separation of metaconid from an- teroconid, ; D, LACM 124117 RM„ ; E, LACM 124123 LM„ heavily worn, S. mascotensis: F, MSU 12531, LM,, from a female, collected 6 mi. W Autlan, 4400 ft, Jahsco, Mexico, LRA2 is more perpendicular to the midline of the tooth and the tip of the meta- conid does not, as a result, appear to extend posteriorly. This may be a function of wear or it could represent a slight morphological change within S. lindsayi populations through time. M2-M3: These teeth do not differ in any appreciable way from those of most cotton rats, such as S. curtisi nd S. hispidus. Reentrant folds are relatively deep and narrow, and the anterior cingulum is moderately to well developed on both teeth, as it is in all cotton rats except S. bakeri and S. peruanus (Martin 1979). M'-M^-M^: Likewise, the upper dentition is not diagnostic. These teeth are relatively large (Table 1), but demonstrate no specific characters which would allow separation from other middle Pleistocene species, such as S. curtisi or S. hudspethensis. Comparisons. —Sigmodon lindsayi was approximately the size of 5. curtisi (Mar- tin 1979, 1986). Utilizing Martin's (1984) formula for estimating body mass in cricetine rodents from M, length, S. lindsayi averaged g, with a range of g. Occlusal length was used for these calculations. The enlarged, symmetrically flattened and laterally extended anteroconid on Ml is a feature that we have seen only on one specimen of the extant Sigmodon mascotensis, the Jaliscan cotton rat. The Mi of MSU 12531, from Jalisco, Mexico, shows a great deal of similarity to the holotype of S. lindsayi (Fig. 3). However, the antero
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