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Show 1896.] OF T H E P A L E O Z O I C OPHIUROIDEA.. 1035 form of narrow bars; also by the absence of the strong marginal plates round the disc. The genus contains two species, S. brisln-golcles (Greg.) and S.leptosoma (Salt.) [13. p. 331, pl. ix. fig. 5], both of which were originally placed in Protaster. The genus differs from Protaster by the family character of having bar-shaped instead of boot-shaped ambulacral ossicles. I have pleasure in naming this genus after Herr Stiirtz, whose careful dissections have added so greatly to our knowledge of the Palaeozoic Ophiurids, and who has previously pointed out [17] that these two species are generically distinct from Protaster. Genus 3. TCENIURA, nov. gen. Diagnosis.-Palaeophiuridae with a small pentagonal disc, not bounded by marginal ossicles. The ambulacral furrow is broad. The oral skeleton is conspicuous and the syngnaths each composed of two separate pieces. The two jaws of each oral angle are closely attached; the mouth-frames are separated and each of them is a short, thick, slightly bent bar. Distribution.-Trenton Limestone, Ottawa. Type species.-Tceniura cylindricus (Billings) [2. pp. 81-82, pl. x. figs. 4 a, 4 6]. Bemarks.-This genus differs from Palceophiura by tbe absence of marginal ossicles from the disc, and from Sturtzura by the smaller size of the disc and the form of the syngnaths. In Sturtzura the jaws end bluntly against a jaw-plate, whereas in Tceniura they appear to taper to a point and have no jaw-plate. This genus is necessary for the second species included by Billings in his genus Tceniaster. The genus was described as discless, which in respect to the type species S. splnosus is correct. In that species the oral skeleton consists of five pairs of large adambulacrals as in ordinary Asterids. The affinities of the true Tceniaster appear to me to be with such forms as Palceaster ruthveni (Forbes) [4. dec. 1, pl. i. fig. 1]. It is asteroid in the oral armature, in its alternately arranged ambulacral ossiclesx, and iu the absence of a disc. I therefore consider Tceniaster a genus of Asteroidea. The second species placed by Billings in this genus has, however, a well-marked disc 2, and has the oral armature composed of five pairs of Ophiuroid syngnaths. It must therefore be included among the Ophuiroidea. Genus 4. EUGASTER, Hall, 1867 [6. p. 290, pl. ix. figs. 7, 8]. Biagnosls.-Palaeophiuridae in which the ambulacral ossicles are subheptagonal in form, the central suture along the arm being zigzag, while the outer angles of the ossicles are cut away for the 1 Billings in his description (2. p. 81) attributes tbe alternation of the ossicles in Tceniaster spinosus to distortion; but this explanation is not consistent with his figure. If distortion had separated the pairs of ambulacral ossicles it ought also to have displaced the syngnaths ; but those of each pair are left precisely opposite. 2 Billings, op. cit. pl. x. fig. 4 a. |