Synthetoceratinae

Syndyoceras is a small extinct genus of Artiodactyla, of the family Protoceratidae, endemic to central North America from the Miocene epoch (23.1—18.5 Ma), existing for approximately .

Lambdoceras is an extinct genus of Protoceratidae belonging to the order Artiodactyla (subfamily Synthetoceratinae) endemic to North America during the Miocene, living epoch 20.6—13.6 Ma, existing for approximately .