Ischyrodonta

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Fossils of Ohio (1996):

• Shell without anterior flap.
• Isochyrodonta is represented by I. elongata Ulrich, which is medium sized, narrowly elongate species having a distinct anterior muscle scar evident on internal molds

Pojeta, Jr. (1971):

• Ischyrodonta is a problematic form, some species of which may possess a duplivincular ligament; if this is so, these species might better be allied to the Cyrtodontidae as suggested by Ulrich (1893 [1895]).

Treatise on Invertebrate Paleontology, Part N, Vol. 1 of 3 (1969):

• Short or elongate, thick-shelled; hinge straight or slightly arcuate, wide and strong; RV with 1 strong cardinal tooth; LV with 2 strong cardinals; no laterals

Ulrich (1893):

• Short or moderately elongate, thick bivalve shells, having small, anteriorly situated beaks, with the hinge line straight or arcuate and extended posteriorly. Hinge plate wide and strong, without posterior lateral teeth, but with two strong cardinal teeth in the left valve, and one large one, and occasionally a small one on each side of it, in the right valve. Ligament internal, posterior to the beaks, linear, supported by from one to three subcardinal ribs. Anterior adductor impression large, deep, subovate, sharply defined on the inner and upper side by a ridge extending from the cardinal teeth to the base of the scar. A small pedal muscle was attached to the under side of the hinge plate immediately above the inner side of the anterior adductor scar. Posterior muscular scar faintly defined, generally but little larger than the anterior scar, situated a short distance beneath the posterior extremity of the hinge. Pallial line simple. Test thick, chiefly calcareous, without the dark epidermis of the Modiolopsidae and Ambonychiidae.
• The relations of this genus are clearly with Ortonella. The reader will find them discussed in the remarks following the description of that genus on the preceding page. Through Ortonella the genus is linked with the Cyrtodontidae, and in referring the species to this family I have been influenced in a great degree by the composition of their shells which seems to be precisely as in Cyrtodonta and related genera, and not as in the true members of the Modiolopsidae. Except for this fact, it would be a difficult matter to draw the line between Ischyrodonta and Modiolodon, the general aspect of the shells and the dentition of their hinges being practically the same in both. Is may seem, therefore, that when casts of the interior only are available it is not possible to discriminate successfully between the two genera. This is, however, not so, since the casts of every species of Ischyrodonta now known exhibit at least one small feature that has not been seen in any species of Modiolodon nor in any now referred to the Modiolopsidae. Namely, the small pedal muscle scar, which in casts, appears as a sharply elevated point situated close to the hinge line between the base of the beak and the upper part of the anterior adductor scar.
• The total number of species now known to belong to this genus is seven. With one exception, I. unionoides, Meek sp., which belongs to the middle beds of the Cincinnati group, all are restricted to the upper one hundred feet of the same formation, leaving over three hundred feet of strata between the known first and second appearance of the genus in which is as yet unknown. Respecting the origin of the Ischyrodonta type of structure, I am obliged to confess total ignorance.

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