Belongs within: Inadunata.
The Icthyocrinidae are a group of crinoids known from the early Silurian to the early Carboniferous in which the crown lacks a marked boundary between cup and arms (Moore 1978).
Characters (from Eckert & Brett 2001): Crown ovoid to pyriform without demarcation between cup and arms in lateral view. Infrabasals typically small, visible in side view or concealed. Radianal situated directly below C ray radial. Anal X, if present, occasionally succeeded by additional plates. Anal sac and interbrachials absent. Arms isotomous, proximal brachials sutured laterally.
Icthyocrinidae [Ichthyocrinacea, Ichthyocrinidae, Ichthyocrinites, Icthyocrinacea]
|--Cleistocrinus Springer 1920EB01
| `--*C. humilis (Angelin 1878) [=Calpiocrinus humilis]M78
|--Synaptocrinus Springer 1920EB01
| `--*S. nuntius (Hall 1862) [=Forbesiocrinus nuntius]M78
|--Paraclidochirus Webster & Fox 1986EB01
| |--‘Clidochirus’ gratiosus Strimple 1963EB01
| `--‘Clidochirus’ keyserensis Stringer 1920EB01
|--Prolixocrinus Eckert & Brett 2001EB01
| |--*P. nodocaudis Eckert & Brett 2001EB01
| `--P. americanus (Springer 1920) [=Clidochirus americanus]EB01
|--Metichthyocrinus Springer 1920EB01
| |--*M. burlingtonensis (Hall 1858) [=Ichthyocrinus burlingtonensis]M78
| |--M. clarkensisM78
| `--M. tiaraeformis (Troost in Hall 1858) [=Ichthyocrinus tiaraeformis, Cyathocrinites tiaraeformis]M78
|--Ichthyocrinus Conrad 1842EB01 [=IcthyocrinusM78]
| |--*I. laevis Conrad 1842 [=Icthyocrinus laevis]M78
| |--I. clintonensis Hall 1852EB01
| `--I. pyriformisM78
`--Clidochirus Angelin 1878EB01 [=Clidocrinus (l. c.)M78]
|--*C. pyrum Angelin 1878EB01
|--C. serrulatus Brower 1973EB01
`--C. springeri Ausich 1984EB01
*Type species of generic name indicated
References
[EB01] Eckert, J. D., & C. E. Brett. 2001. Early Silurian (Llandovery) crinoids from the Lower Clinton Group, western New York State. Bulletins of American Paleontology 360: 1–88.
[M78] Moore, R. C. 1978. Flexibilia. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2 pp. T759–T812. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).