American species of _Microtus_ (genus and subgenus) in which the lateral ossifications are weakly developed or absent (although cartilaginous lateral processes are present) include _M. mexicanus_ and _M. californicus_. In other subgenera, species of _Microtus_ having reduced lateral ossifications are _M. (Pedomys) ochrogaster_, _M. (Pitymys) pinetorum_, _M. (Pitymys) parvulus_, _M. (Pitymys) quasiater_, _M. (Arvicola) richardsoni_, and _M. (Stenocranius) miurus_.

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