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“15. Stephanocyathus platypus (Moseley, 1876),
Plate 7, figs. 3-6
Ceratotrochus platypus Moseley, 1876, p. 554.
Stephanotrochus platypus; Moseley, 1881 p 154, Jourdan, 1895, pp. 19, 20 (is S. nobilis (Moseley, 1873)).
Not Stephanotrochus diadema var. platypus; Gravier, 1920, pp. 46, 47 (is S. moseleyanus (Sclater, 1886)
Stephanocyathus sp. Squires and Ralph, 1965, pp. 262, 263, figs. 3, 4.
Stephanocyathus (S.) sp. Squires and Keyes, 1967, p. 24, pl. 2, figs. 11, 12.
Stephanocyathus platypus; Zibrowius, 1980, p. 97.
Description. Corallum free, bowl shaped, up to 75 mm in CD and 30 mm tall. Theca initially flat to slightly concave up to CD of 35-40 mm, then calicular edges turn upward rather abruptly and continue to grow at an angle of 60°-70° from horizontal. Costae not prominent on horizontal section, but C1 and C2 usually sharply ridged on upturned peripheral theca. Theca, except for C1 and C2, covered by very fine, low, rounded granules.
Septa hexamerally arranged in five cycles; S6 common in larger coralla of up to 115 septa. S1 extraordinarily exsert in form of rounded lobe projecting up to 16 mm beyond theca. S2 also highly exsert; remaining septa barely exsert, only those flanking S1 and S2 rising higher than S3. Calicular margin scalloped, apices corresponding to S1 and S2. S1 extending to center of calice, there considerably thickened and fused into rudimentary columella. S2 reaching almost to center and joining in fusion. S3 falling just short of fusion and terminating in slightly lobed free end. S4 slightly smaller than S3 and also bearing small, broad paliform lobe, this lobe bending toward adjacent S3 but rarely fusing with it. Where pairs of S6 present, enclosed S5 enlarged to almost size of S4 and also bearing small lobe bending toward adjacent S4. Normally, S5 and all S6 short, extending only one third of distance to center. S3, S4, and enlarged S5, bearing broad, low paliform lobes not separated by notches; S1 and S2, however without lobes and usually uniformly concave below level of theca. Septa straight with smooth inner edges. Septal granules low, blunt, and arranged in poorly defined lines.
Discussion. There is little doubt that Squires and Ralph's  Stephanocyathus sp. And Moseley’s S. platypus are identical. Moseley's  original description was based on two small specimens with flat bases and calicular edges that had not yet turned upward; Squires and Ralph's very large specimen had an originally flat base that had subsequently become deeply bowl shaped. The three Eltanin specimens confirm the continuity of the ontogeny. S. diadema [Moseley, 1876) also has a flat base as a juvenile, which, like that of S. platypus, curves upward with greater size. Likewise, the base of S. laevifundus Cairns, 1979, is usually flat but is sometimes gently bowl shaped.
S. platypus is most similar to S. moseleyanus (Sclater, 1886) from the northeast Atlantic especially in shape and septal exsertness. The latter is distinguished by its papillose columella, P1 and P2, and septal junctions near the columella.
Material. Eltanin sta. 1718 (2), USNM 47522; sta. 1818 (1), USNM 47423. Syntypes.
Types. Two syntypes of S. platypus, collected at Challenger station 164, are deposited at the British Museum (18126.96.36.199). Type-locality: 34°13’S, 151°38’E (off Sydney, Australia); 750 m.
Distribution. Known from only four records from off Sydney, Australia; off New Zealand; and from a seamount (Eltanin station 1718) east of New Zealand (Map 5). Depth range: 622-913 m. Like the previous species, it does not occur in the Subantarctic region as defined by Hedgpeth  but is included here because of its proximity to the Subantarctic region.”