Mesoplodon perrini Dalebout, Mead, Baker, Baker & van Helden 2002

Head Shape The rostrum is relatively short compared to all other species in the genus, except, M. hectori and M. peruvianus. The melon forms a small bulge. The mouthline is straight. Coloration Adult males are dark gray dorsally and grade to white ventrally. The ventral side of the tail flukes are pale gray with converging striations. There is a white patch around the umbilical scar. White linear scars are present on adult males, but appear to have been made by single tooth. Coloration of adult females has not been described. Size Adult body length range is unknown. Recorded maximum body length for adult males and females is 3.9 m and 4.4 m, respectively. Length at birth is 2.1 m. Most Likely Confused With: Mesoplodon peruvianus

Diagnostic features of the skull and mandible On the vertex of the dorsal skull the premaxillary bone extends forward of the nasal and frontal bones. Separates from Berardius and Ziphius. A sulcus (groove) running along the middle of the combined surfaces of the nasal bones so depresses their combined 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 are positioned at the apex of the mandible.Separates from M. bidens, M. bowdoini, M. carlhubbsi, M. densirostris, M. europaeus, M. grayi, M. ginkgodens, M. layardii, M. peruvianus, M. stejnegeri, and M. traversii. The space between the nasals is narrow and reversed "V" shaped Separates from M. hectori and M. mirus.

Stranding Distribution Stranding records occur from Monterey to San Diego in California, but oval scarring typically of cookie cutter sharks indicates that species may exist in more southern waters preferred by Isistius. It may also be possible that Isistius has a more northern range than is currently reported.

Tooth position A single pair of teeth are positioned sub-terminally at the apex of the mandible. Tooth exposure Nearly all of erupted teeth in adult males are exposed above the gumline. Teeth do not erupt in females or juveniles. Tooth shape The shape of the exposed tooth is a rough isosceles triangle, but with a smoothly convex anterior margin. Angle formed by the denticle is 60 to 65 degrees. Dimensions for an adult male tooth measured 64 mm long, 47 mm wide, and 12 mm broad.