The shells from Tonga are damaged and collected dead but are still identifiable. They have more slender whorls and a higher spire than the holotype, but are otherwise similar to it. The last teleoconch whorl bears 3 wide, thin and fragile, wing-like varices. It resembles Pterynotus bednalli (Brazier, 1878) but differs in the ornamentation of the first teleoconch whorls. Pterynotus bednalli bears 6 or 7 thin axial lamellae on the first and second whorls, and 3 lamellae from the third whorl onwards. Pterynotus marshalli bears 3 thin varices and a small lamellate ridge on its first whorl, and from its second whorl onwards has 3 thin varices and a small axial ridge. Pterynotus bednalli also has more numerous secondary and tertiary spiral cords on its last teleoconch whorl.