Hemistreptocrinida

Cup of Paractocrinus tuberculatus in (clockwise from left) lateral, base and summit view, from Moore et al. (1978).

Belongs within: Crinoidea.

The Hemistreptocrinida are an Ordovician group of small crinoids with plate circlets in the cups only slightly offset (Moore et al. 1978).

Characters (from Moore et al. 1978, as Paractocrinidae): Small crinoid cups composed of three or more circlets of plates which are only slightly offset rather than alternating; anal plates absent; proximal columnal sometimes cone-shaped with no facet for additional columnals.

<==Hemistreptocrinida [Hemistreptocrinoidea]
    |--Paractocrinus Jaekel 1918 [Paractocrinidae] ML78
    |    |--*P. tuberculatus Jaekel 1918 ML78
    |    `--P. laevis Jaekel 1918 ML78
    `--Hemistreptocrinidae ML78
         |--Hemistreptocrinus Arendt 1976 ML78
         |--Nonparactocrinus Arendt 1976 ML78
         |--Parorthocrinus Jaekel 1918 ML78
         |    `--*P. liber Jaekel 1918 ML78
         `--Tetractocrinus Jaekel 1918 (n. d.) ML78
              `--*T. compactus Jaekel 1918 J18

*Type species of generic name indicated

REFERENCES

[J18] Jaekel, O. 1918. Phylogenie und System der Pelmatozoen. Paläontologische Zeitschrift 3: 1–128.

[ML78] Moore, R. C., N. G. Lane, H. L. Strimple, J. Sprinkle & R. O. Fay. 1978. Inadunata. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2 pp. T520–T759. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).