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Show 1036 DR. J. W. GREGORY ON THE CLASSIFICATION [Dec. 15, reception of the podia. The adambulacral ossicles have a flat base, and thence bend forward crescentically. The mouth-frames are massive, and those of each pair meet along the middle line of the oral angle. Distribution.-Hamilton Series, Middle Devonian, Madeson County, N e w York. Type species.-Eugaster loganl, Hall. Bemarks.-This genus I only know from Prof. Jas. Hall's figures, and in spite of their clearness I feel much doubt as to the wisdom of diagnosing it from these alone. Its affinities are clearly with the Palaeophiuridae, but it approaches the Protasteridae in one respect: for, owing to the deep depressions in the outer angles of the ambulacral ossicles, the outer side forms a short rudimentary wing. The genus differs from all the Protasteridae by the absence of a muscular groove across the ambulacral ossicles. Among the Palaeophiuridae it differs from Palceophlura by the absence of marginal ossicles, and from Sturtzura and Tceniura by the massive nature of the mouth-frames. Genus 5. PTILONASTER, Hall, 1867. Distribution.-Chemung Series, Upper Devonian, Cortlandville, N e w Tork State. Type species.-Ptilonaster prlnceps, Hall, 1867 [6. p. 292, pl. ix. fig. 9]. Bemarks.-This genus is an ally of Eugaster, as Liitken [9. pt. hi. p. 82] has already remarked ; it is, however, generically distinct. I only know it from Hall's figures, and therefore prefer to leave the preparation of a formal diagnosis to an American palaeontologist. Order II. STEEPTOPHIDE^E, Bell, 1892. Diagnosis.-Ophiuroida in which the ambulacral ossicles are opposite and are generally fused into vertebral ossicles. In such cases the vertebral ossicles articulate by a more or less simple ball-and- socket joint. The covering-plates of the arms are more or less regularly developed, and consist of a superior, an inferior, and a pair of lateral arm-plates to each segment. The lateral arm-plates generally bear spines. Remarks.-The main character of this order is that the ambulacral ossicles are paired, but primitive. The order differs from the preceding by having the vertebral ossicles always opposite instead of alternate. In some of the simplest members of the order, as Ophiurlna, the ossicles are not fused, there are no ventral arm-shields, and an ambulacral furrow runs along the ventral side of the arm. In the next higher family, as in the genus Lapworthura, the ambulacral ossicles are fused, but have plain articulating surfaces, and there is an ambulacral furrow. In recent members of the group the vertebral ossicles are of a more complex type, but the articulating surfaces are streptospondyline; in some, such as |