Diagnostic features of the skull and mandible
On the vertex of the dorsal skull, the premaxillary bone extends forward of the nasal and frontal. Separates from Berardius and Ziphius.
A sulcus (groove) running along the middle of the combined surfaces of the nasal bones so depresses their middle that it is the lateral portion of each nasal bone that reaches farthest forward on the vertex. Separates from Tasmacetus and Indopacetus.
When the skull is upright and the long axis of the anterior half of the beak is horizontal, a horizontal plane transecting the summit of either maxillary prominence transects the mesethmoid bone. Separates from Hyperoodon.
Tooth alveoli of mandible overlap or are positioned slightly anterior to the mandibular symphysis. Separates from Berardius, Ziphius, Tasmacetus, Indopacetus, Hyperoodon, M. hectori, M. mirus, M. perrini, M. densirostris, M. ginkgodens, M. peruvianus, and M. stejnegeri
Basirostral groove absent or present as a shallow groove that does not extend past the prominental notch. Separates from M. grayi and M. layardii.
The maxillary prominences rise less than 10 mm higher than the height of the premaxillaries where the latter passes between the former. Separates from M. bowdoini, M. carlhubbsi, and M. europaeus.