Phylum: Mollusca
Class: Bivalvia
Order: Actinodontida
Family: Cycloconchidae
Genus: Cycloconcha Miller, 1874
Cincinnatian Species: Cycloconcha milleri

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Geologic Range
Middle Ordovician – Late Ordovician

Common Paleoecology
Cycloconcha is an extinct genus of passively mobile infaunal suspension feeders

Identification in Hand Sample:

  • Equivalve shell
  • Concentric growth lamellae
  • Adductor muscle scars sub-equal
  • Well-defined cardinal, anterior lateral, and posterior lateral teeth
  • 2 or 3 teeth, reach or nearly reach the beak

Geographic Occurrences

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Morris and Eagar:

  • Shell shape: equivalve, orbicular disk-shaped umbones prosogyral with slight lunule.
  • Ornament: concentric growth lamellae.
  • Hinge: ligament groove at dorsal shell margins, posterior to umbones, nymph not clearly delimited. Teeth radiate from initial point below umbones, all are cardinals. Fairly numerous teeth present.
  • Muscle scars: adductors sub-equal, posterior slightly larger than anterior. Anterior adductor well inset. Pallial line simple and entire. Pedal attachment scars multiply paired and sub-equal, grouping almost regular.

Pojeta, Jr. & Runnegar (1985):

  • Cycloconcha has a full complement of anterior and posterior pseudolateral teeth and pseudocardinal teeth, a small external ligament, and several small pedal muscle insertions between the abductor muscle scars.

Pojeta, Jr. (1971):

  • Cycloconcha possesses well-defined cardinal, anterior lateral, and posterior lateral teeth; the teeth are numerous and all reach or nearly reach the beaks. In addition, the genus is elongate in an anterior-posterior direction and has an external, elongate ligament posterior to the prosogyral beaks, an integropalliate pallial line, and multiple pedal dears extending between the adductors.
  • Cycloconcha is similar in general morphology and especially in hinge features to such late Paleozoic crassatellaceans as Oriocrassatella, and it seems likely that the crassatellaceans are descended from the cycloconchids.
  • The general morphology and shell shape of the equivalved, elongated telliniform genus Cycloconcha suggests that it was a shallow infaunal form. There is nothing to suggest that Cycloconcha is closely related to Tellinacea, rather it seems to be related to the Paleozoic crassatellaceans.

Treatise on Invertebrate Paleontology, Part N, Mollusca 6(1 of 3) (1969):

  • Subcircular, with 2 or 3 cardinal teeth near middle of hinge and well-differentiated, long, lateral tooth in front and behind beaks.

Miller (1874):

  • Shell equivalve, equilateral or subequilateral, circular or subcircular in outline, and marked concentrically. Hinge line having the cardinal teeth in the middle, with a lateral tooth of about the same length on each side.

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C. milleri