The similarities or differences in the baculum in Old World and New World representatives placed in the same genus or subgenus, or thought to be "companion species" have been commented upon in accounts of _Lemmus_, _Dicrostonyx_, _Clethrionomys_, _Lagurus_, _Arvicola_, _Stenocranius_, _Chilotus_, _Chionomys_, _Pitymys_, and in accounts of _Microtus agrestis_ as compared with _M. pennsylvanicus_, and _Microtus oeconomus_ (both Old World and New World).
The baculum in the Microtinae more closely resembles the baculum in the Cricetinae of the Old World than in the Murinae, or than in any other rodents known to me. This resemblance suggests relationship between Microtinae and Cricetinae.
LITERATURE CITED
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ZIMMERMAN, K.