Shell rather like P. zelandica (Q. & G.), but much smaller and with stouter spines. P. zelandica normally has 3 subsidiary rows of spines across the body whorl (2 in juveniles) where P. kopua has none. The protoconch is large in comparison with that of zelandica. Although the spines are formed in essentially the same way in the two species, i.e., by the bending over of a lamella so that the edges join proximally but remain open distally, the bases are much wider in kopua. The spines are less numerous in kopua (5 on the body whorl compared with 8 in zelandica of comparable size). The aperture is very similar, with the remains of earlier canals forming a series of spines down the neck. Height. 19,3 mm.; diameter, 15,0 mm. (including spines ), 8,0 mm., body whorl alone.