Middle Ordovician – Late Ordovician
Iocrinus is an extinct genus of stationary intermediate-level epifaunal suspension feeder
Identification in Hand Sample:
- Conical cup with 5 basals
- Robust body, but rather small compared with the arms and column, wider above than the length from the base to the summit of the first radials
- Tall anal sac
- Arms branching isotomously up to 8 times
- Column transversely pentalobate
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- Cup conical, basals 5; radials large, supporting several primibrachs in each ray. Infer- and superradial in C ray only; superradial axillary, at level of first primibrachs, suppporting arm on right side and anal sac plates on left. Anal sac tall, with row and large anal plates on posterior side and thin, horizontally plicate plates laterally. Arms branching isotomously up to 8 times. Column transversely pentalobate.
Meek & Worthen (1865):
- Body robust, but rather small compared with the arms and column, wider above than the length from the base to the summit of the first radials ; distinctly truncated at its connection with the column, from which point the sides expand rather distinctly upwards ; subpentagonal in outline as seen from below. Basal pieces pentagonal, wider than long, and all excavated or- indented on the outside at the superior angle and down the middle. First radial pieces longer than the basal, about three-fourths as long as wide, broadly truncated above, and regularly pentagonal in form, excepting two on the anal side, which appear to each have one of the superior lateral angles a little truncated for the reception of a small anal piece ; all deeply indented at their inferior lateral angles, so as to leave a broad, rounded, undefined ridge or prominence descending from the middle to the basal pieces. Succeeding radial pieces forming free arms, nearly as wide as, but much shorter than, the first; in four of the rays all transversely oblong, and about three or four times as wide as long, excepting the fourth or fifth pieces, which is pentagonal, and supports, on its sloping upper side, the first divisions. In one ray on the anal side the second piece is pentagonal, larger than that of any of the others, and gives off a lateral branch from its short sloping side on the left, above which the other pieces present the same size and form seen in the other rays. Arms after the first division on last radial, bifurcating again on the sixth or seventh piece, after which they are known to divide again in one arm, on the sixth piece, which is as far as our specimen shows the suture.
- Surface usually appears smooth, but sometimes showing traces of scattering granules. Column comparatively large, distinctly pentagonal, and expanding upwards near the base of the body, where it is composed of irregularly alternating thicker and thinner segments; central perforation small and round.
- Height of body from base to summit of first radial pieces, .35 inches; breadth at summit of first radials, about .67 inches; length of five succeeding radial pieces, .46 inches; breadth of do. about .15 inch. Breadth of column at its connection with the base, .32 inches.
- This is perhaps the largest and most robust species of this genus known. It is composed of thick, strong plates, and the indentions or excavations at the points where the superior angle of each basal plate connects with the inferior lateral angles of the first radials, together with the more shallow depressions extending down from these points to the lower margin of the basal pieces, give a pentagonal outline to the body- the five angles being coincident with those of the column.
- Locality-Cincinnati, Ohio. Cincinnatian group of Lower Silurian.