Lyrodesma major

Phylum: Mollusca
Class: Bivalvia
Order: Trigonioda
Family: Lyrodesmatidae
Genus: Lyrodesma
Species: Lyrodesma major Pojeta, 1971

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Taxonomic History:

  • 1879 Cleidophorus major Ulrich, Jour. Cincinnati Soc. Nat. Hist., 2, p. 25, pl. 7, fig. 23.
  • 1894 Lyrodesma major Ulrich, Geol. Minnesota, 3, pt. 2, p. 611, fig. 45, a-g.
  • 1879 Lyrodesma major Foerste, Upper Ordovician faunas of Ontario and Quebec, p. 171, pl. 43, fig. 6.

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Stratigraphic Occurrences


Geographic Occurrences

Map point data provided by iDigBio.
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Sequences (Formations)

  • C5 Sequence (Waynesville)
  • C3 Sequence (Corryville)

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Identification in Hand Sample

  • Small, moderately convex shell
  • Post-umbonal slope marked by twelve or more, fine, radiating striae
  • Elevated beaks
  • Prominent, rounded umbones
  • Interior presents a deep triangular groove anterior to the beaks

Lyrodesma major from unknown formation of Hamilton County, Ohio (CMCIP 51425)

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Davis (1998):

  • Pelecypod. Internal mold. Shell small, moderately convex. Note the prominent beak areas. Corryville and Waynesville.

Foerste (1924):

  • Anterior margin rounded most rapidly at mid-height of shell, slightly convex above this level as far as the beak. Umbonal part rather broadly rounded along the cardinal outline. Posterior to the beak outline of the shell gently concave, inclining downward so as to produce an attenuated posterior end. Umbonal ridge only moderately divergent from the cardinal line, distinct near the beak but more or less rounded into the general convexity of the valve posteriorly. Post-umbonal slope marked by twelve or more, fine, radiating striae, leaving a narrow area along the hinge-line unmarked.
  • Among a group of specimens found in the Proetus horizon at Chambly Canton, on Richelieu river, is one specimen very similar to the specimens figured and described by Ulrich, especially in the rounding of the umbonal ridge posteriorly. The shell, however, is shorter; the posterior outline less strongly rounded; the radiating striae on the post-umbonal slope slightly coarser, numbering about ten, and there appears to be a coarse clavicular ridge 1.5 mm in front of the beak, slanting backward at an angle of 85 degrees with the horizontal line In the associated specimens, the radiating striae on the post-umbonal slope are equally fine, but the umbonal ridge much more distinctly angulate, as in the species described next, L. huguesensis.
  • Locality and Horizon. Described originally from the Corryville member of the Maysville at Cincinnati, Ohio. More extensively described and figured later from the Waynesville, Clarksville, Ohio.

Ulrich (1879):
With regard to then Cleidophorus major

  • Shell sub-ovate, ventricose, height and length respectively at 5 ½ to 10; umbones rounded and prominent; beaks elevated, sub-acute about one-fourth the entire length of the shell from the anterior margin; posterior cardinal line nearly straight, but a strongly defined sharp, umbonal ridge extending from the beaks and gradually becoming obsolete toward the posterior margin; the space between the ridge and cardinal line slightly depressed; anterior margin rounded; basal margin broadly, but regularly rounded; posterior end rather narrowly rounded.
  • Surface unknown. The interior, as shown in casts, presents a deep triangular groove just anterior to the beaks, and passing, for a distance a little more than one-third of the heighth of the shell, toward the anterior basal margin; the anterior muscular scar is distinctly defined, ovate, and situated immediately in advance of the groove; the posterior scar is rather large, sub-circular, and placed on the umbonal ridge (in the cast of the interior) about in the middle of the distance from the beaks to the posterior basal margin; pallial line obscure, simple, and running nearly parallel with, and considerably within, the basal border.
  • This is the largest species of the genus.
  • It is most nearly related to C. elongatus, Hall, from Nova Scotia. That shell however is sinuate, has the cardinal margin rounded and not straight, the beaks closer to the anterior margin, and has the clavicle bent and proportionally longer than in our species.

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