Seven species of the equid genus Mesohippus have been recognized in the Cypress Hills fauna, all based on isolated cheek teeth. Additional teeth collected in recent years permit a reduction to four species: M. westonii (Cope), small size, vestigial hypostyle; M. propinquus Lambe, small size, prominent hypostyle; M. stenolophus Lambe, medium size, prominent hypostyle; M. assiniboiensis Lambe, large size, well developed hypostyle. The species M. praecocidens Lambe and M. planidens Lambe are regarded as undeterminable. The specimen identified by Lambe as M. brachystylus Osborn may belong to M. stenolophus.

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