Fig. 6

a–d Tremaster mirabilis Verrill, 1879. a, b Large individual in abactinal a and actinal b aspects (original of Clarke and Downey 1992, pl. 44, Figs. a, b), Newfoundlands Banks, NHMUK Zoology 1968.4.11.11. c, d Drawings of dissected T. mirabilis (after Jangoux 1982, fig. 1c, d). c Shows the internal view of the chevron-plate-lined duct leading from the actinal to abactinal surface. d shows the denuded actinal surface with two large oral ossicles (bottom) and the opening of the internal duct. e Cast of the unique holotype of Protremaster uniserialis Smith and Tranter, 1985. Note that the specimen has folded obliquely and shows both actinal and abactinal surfaces. Sinemurian, Lower Jurassic, Alexander Island, Antarctic Peninsula, BAS KG 2986.17. f Actinal aspect of interradius, holotype of Mesotremaster zbindeni Hess, 1981. The chevron plate-lined duct is exposed on the actinal surface (cp). Figured after Hess 1981, pl. 2), holotype, NMB no. M9550, middle Oxfordian, Ruchenette, Switzerland. Scale bars equal 5 mm