Pliotrema Regan.

Weigmann, Simon, Gon, Ofer, Leeney, Ruth H., Barrowclift, Ellen, Berggren, Per, Jiddawi, Narriman & Temple, Andrew J., 2020, Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan, PLOS ONE 15 (3), pp. 1-56 : 5

Genus Pliotrema Regan.

Type species. Pliotrema warreni Regan by original designation.

Revised generic diagnosis of Pliotrema Regan. Pristiophoriform sharks are characterized by a flat, greatly elongated, and saw-like snout with long ventral barbels and closely-set rows of lateral teeth and ventral spines, as well as anterior-most basiventral cartilages that are laterally expanded and have curved, dorsally reflected margins. The teeth on the lateral edges of the snout are alternately large and small in juveniles but the number of small teeth in the interspaces between large teeth increases ontogenetically. The individual rostral teeth are fixed to the dermis and not embedded in sockets. The ventral spines are more or less pronounced and decrease in size and partially get lost with age, in some species they are colored black. In embryos, the lateral rostral teeth and ventral rostral spines are folded back beneath the skin. Oral teeth small, with conical cusp on broad base, arranged in several series.

The genus Pliotrema is characterized by the possession of six gill slits and serrated large lateral rostral teeth. Like Pristiophorus , Pliotrema lacks nictitating lower eyelids, has large spiracles situated behind the eyes, two dorsal fins without spines–the first in front of pelvic fins, no anal fin, no precaudal pit, and a caudal fin with subterminal notch and strongly reduced lower lobe. In Pliotrema , barbel origin to anterior nostrils 1.4–2.3 times anterior nostrils to symphysis upper jaw; prenarial length 1.3–1.7 times prebarbel length; preoral length 1.5–2.7 times interdorsal space; pectoral-fin anterior margin 1.2–1.6 times dorsal– caudal space; mouth width 2.7–6.6 times spiracle length. First dorsal fin originates about opposite pectoral-fin free rear tips. Lateral trunk dermal denticles tricuspidate, rather flat and imbricated. Color uniform pale to dark brown dorsally, sometimes with one or two weak to rather pronounced yellowish longitudinal stripes, partially with whitish fin margins (particularly caudal and pectoral fins); ventrally whitish, partially with sparse darker mottling; dorsal rostrum surface with two distinct longitudinal dark stripes, lateral rostral teeth edged dark. Monospondylous centra 52–57; precaudal diplospondylous centra 46–56; total vertebral centra 151–164.