North American index fossils, invertebrates . e a second row of 10 or 12 radial and interradial 204. A. scutelliformis Hall. about one and one half inches. (Type of genus.)Helderbergian of New York, etc. (Characteristic of Upper New Scotland and Becraft limestones.) LXXX. Ancyrocrinus Hall. Form bulb-shaped, with lateral processes and a central column. Represents the lower end of a crinoidal stem, the lateral appen-dages being a kind of radicular cirri used in anchoring the formwhich is probably free-floating. It may later become detached,leaving the adult free
North American index fossils, invertebrates . e a second row of 10 or 12 radial and interradial 204. A. scutelliformis Hall. about one and one half inches. (Type of genus.)Helderbergian of New York, etc. (Characteristic of Upper New Scotland and Becraft limestones.) LXXX. Ancyrocrinus Hall. Form bulb-shaped, with lateral processes and a central column. Represents the lower end of a crinoidal stem, the lateral appen-dages being a kind of radicular cirri used in anchoring the formwhich is probably free-floating. It may later become detached,leaving the adult free. Devonic. 205. A. spinosus Hall. (Fig. 1908.) part of bulb broadly rounded; arms short. Onondaga of Falls of the Ohio. 206. A. bulbosus Hall. frotp A. spinosus in its longer and less ventricose basal portion, its longer and less diverging arms. The column above the 570 NORTH AMERICAN INDEX FOSSILS, bulb often elongated, round in the lower part, obtusely quadran-gular above. (Type of genus.)Hamilton of New Fig. 1908. Ancyrocrinus spinosus, lateral and summit views. (After Hall.) Branch ASTEROZOA I. OPHIUROIDEA Gray. The Ophiurians, or brittle stars, are marine echinoderms abun-dantly represented in the modern fauna. They appear as early asthe Siluric and have representatives throughout the Palaeozoic. Amore or less sharply defined central diskcontains the mouth and digestive cavity,which latter does not extend into the slen-der, rounded arms. The arms consist of anaxis of jointed calcareous disks (vertebralossicles), surrounded by plates or a leatheryintegument and destitute of open ambu-lacral grooves. The madreporic body issituated on the oral (actinal) side of thedisk. The arms are movable and serve for
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