- 1893 Anoptera Ulrich, Geol. Surv. Ohio, Rep., vol. 7, p. 649.
- 1918 Cleionychia Foerste, Ottawa Naturalist, vol. 31, p. 121.
- 1966 Anoptera
Middle Ordovician – Late Ordovician
Cleionychia is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample:
- 1 cardinal tooth in some species
- Concentric growth lines
- Prominent byssal sinus
- Shell equivalved, inequilateral, rectangular
Treatise on Invertebrate Paleontology, Part N, Vol. 1 of 3 (1969):
- Similar to Ambonychia, but without radial ornamentation or byssal opening; 1 cardinal tooth just under beak in some species, no posterior laterals.
- Small Ambonychiidae with concentric growth lines, prominent byssal sinus, and with the height greater than the length. Shell equivalved, inequilateral, rectangular and erect, and lacking an anterior lobe; beaks terminal and prosogroval, project far forward; prosopon composed of concentric growth lines only, these are nimierous, and almost imbricating; only outer shell layer known, indicating a two-layer shell. No discernible byssal gajje; byssal sinus prominent, size small, height greater than length, and approaching the diagonal dimension. Internal and ligamental features unknown except for the presence of a bifid anterior byssal retractro scar in one species.
- During the present year I succeeded in obtaining two species from the rocks of the Cincinnati group that seem to belong to this genus. One is from the lower beds (Utica horizon) and of considerable interest because of its oblique shape and consequent approximation toward the upper Silurian and Devonian genus Mytilarca. The other is from the upper beds of the formation and has an erect form like the typical lower Trenton species of the genus.
- Shell less than the average size for the genus, moderately convex, oblique, the outline obscurely rhomboidal; anterior and basal margins gently rounded, oblique, post-ventral extremity strongly rounded, posterior margin but little convex; hinge line straight, about half as long as the greatest oblique diameter of the shell; post-cardinal angle about 125 degrees, antero-cardinal angle between 65 and 70 degrees. Beaks moderately prominent, not strongly incurved. Surface marked with fine concentric lines of growth and irregular concentric undulations, generally strongest on the anterior slope. Interior unknown.
- This species is smaller and more oblique than any other now referred to the genus. C. rhomboidea Ulrich, of the lower Trenton (Birdseye limestone) in Minnesota, is the most like it so far as shape is concerned, but the concentric undulations are believed to indicate closer affinities with C. undata Emmons, sp. In the latter the surface undulations are much broader, the hinge longer, and the shell much less oblique.