==============+===========+=========+==========+===========+========== | Number of | Average | 3 x | 1 | Species | specimens | length | standard | standard | Range | | | error | deviation | --------------+-----------+---------+----------+-----------+---------- _B. taylori_ | 108 | 2.535 | .078 | .274 | 2.00-3.12 | | | | | _B. musculus_ | 58 | 3.324 | .090 | .233 | 2.80-3.88 --------------+-----------+---------+----------+-----------+---------- In each of the two species, individual and geographic variation in the baculum is slight; its length varies insignificantly according to age. Excluding juveniles contained in Table 4, but including young and subadults, only three bacula of _B. taylori_ were longer than 3 mm., and only one baculum of _B. musculus_ (a young) was shorter than 3 mm. The total length of the baculum, considered together with its shape, serves to identify to species all specimens examined by me. The bacula of both species of _Baiomys_ were compared with bacula of _Akodon_, _Scotinomys_, _Holochilus_, _Oryzomys_, _Zygodontomys_, _Reithrodontomys_, _Thaptomys_, and _Calomys_ and illustrations of bacula by Blair (1942:197, 200) of _Peromyscus_ (subgenera _Peromyscus_, _Haplomylomys_, _Podomys_), _Ochrotomys_, and material at the University of Kansas Museum of Natural History of _Megadontomys_. Shape of baculum most resembled that of _Ochrotomys_ and _Calomys_. The bacula of _Baiomys_, as pointed out by Blair (_op cit._:203), differ as much from those of the genus _Peromyscus_ as do the bacula of _Reithrodontomys_ and _Onychomys_. In size of baculum, _Baiomys_ resembles _Ochrotomys_. Blair (_op. cit._:202) pointed out that the length of the baculum of _B. taylori subater_ was contained in the length of the animal's body 20.3 times, and 24.2 times in the length of that of _Ochrotomys nuttalli_. The length of the baculum of _B. musculus_ (average of 58 specimens without regard to subspecies) is contained in the length of the body (of specimens from which the bacula were removed) 22.7 times, a figure approaching that in _Ochrotomys_. When bacula of both species of _Baiomys_ were compared to those of _O. nuttalli_, bacula of _B. musculus_ were found to most closely resemble those of _O. nuttalli_. The baculum of a single specimen of _Calomys_ (_C. laucha_) was contained in the length of the body 15.5 times. In general shape, as well as in possession of an anterior knob and the position of the expanded posterior wings, the baculum of _C. laucha_ resembles the baculum of _Ochrotomys_ and _Baiomys musculus_.