The present species is usually known as Trophon vaginatus (De Cristofori &. Jan, 1832), as far as specimens from the shelf are concerned. We have examined the lectotype in Museo Civico di Storia Naturale, Milano. It was described as a Pliocene fossil from Tabiano, N. Italy. It is certainly quite similar to the Recent Mediterranean form , but there is a distinct difference in the larval shell, which in the Recent form consists of about 1.5 whorls while the Pliocene form has more than 2.5 whorls, and possibly had planktotrophic larval development. This observation was confirmed by examining numerous specimens from other N Italian Pliocene deposits (in the rich collection of Delia Bella). We therefore consider the Recent form specifically distinct from the Pliocene fossil.

The sculpture of P. echinata is variable and this does not facilitate identification. A few features are, however, quite constant: paucispiral larval shell, long slender siphonal canal and strongly-shouldered whorls. T. dabneyi has a larval shell of 2.5 brownish whorls while that of P. echinata has 1.5 whorls and is colourless. It also has more rounded whorls and more solid, rib-like axial sculpture, not thin lamellae as P. echinata. T. droueti has a shorter siphonal canal and more pronounced spiral sculpture, approximately as strong as the axial one. T. barvicensis has a shorter siphonal canal, stronger spiral sculpture and a higher shoulder of the whorls.
The Caribbean T. aculeatus Watson, 1883 (= lacunella Dall, 1889) resembles P. echinata, especially specimens from intermediate depths, but is higher, more slender and has a deeper suture than echinatus. We have not seen specimens of aculeatus with spiral sculpture.
T. abyssorum Verrill, 1885 (? = limicola Dall, 1902) resembles the cossmanni "form", in the shape of the shell and the sculpture. It is known from off the NE U.S. from 1500 to 3500 m. The available material of that species, however, is scanty and there is a large gap in the distribution in the N Atlantic, from Rockall to U.S., where no Trophon species related to these species are known. Therefore we find it better to keep them separate.
More problems arise when going further south. Trophon guineensis Thiele was described from the Gulf of Guinea (03° 10 N, 05°28 E, 2278 m). The larval shell is not known, but the postlarval shell can hardly be distinguished from the cossmanni "form" We have examined two specimens from off Angola (WALDA st CY15, 10°30 S, 11°55 E, 1756 m). The shell is identical, but the larval shell is destroyed.
Further to the south we have examined many specimens from the sides of the Walvis Ridge (WALVIS st CP13, 32°18 S, 13°16 E, 3550 m and CP 14, 32°29 S, 13°25 E, 3675 m). They were all heavily encrusted by zoantharians and the larval shells were badly corroded. They had normal eyes, but no eggs were observed. They resemble T. acceptans Barnard, 1959 (see also 1963), described from 33°34 S, 1160 — 3275 m. Barnard (1963) doubted the validity of this species and said "I fully expect that acceptans will not be accepted when further material is obtained from the Atlantic trough along the west coast of Africa" It is still impossible to say anything about the validity of these two names, but the differences in the development of the eyes indicate that they may not be synonyms.

Distribution: The continental shelf of the Mediterranean and the E Atlantic north to the Bay of Biscay (typical echinatus), and the continental slopes of the same area and north to the Rockall Trough (Houart, 1981 as T. cossmanni and Nodulotrophon scolopux) (cossmanni "form"). Also present in the bathyal zone of the Azores (cossmanni "form").