The term aptychus is from the Greek meaning a body apparently folded into two parts. Aptychi are symmetrical pairs of calcareous plates which in vivo were in contact along their hinge-like straight line of symmetry. They are often found close to or inside the shells of ammonoid cephalopods, and sometimes even closing the aperture of the shell.
Superficially, aptychi resemble bivalve shells, in that their concave surface is smooth and the convex surface is ornamented in various ways: granulated, ribbed, furrowed, or punctate (Fig. 1). Between the outer and inner layers, a middle layer with a cellular or tubular structure may be found. If aptychi are completely preserved, a thin dark layer of horny or chitinous material covers the innermost calcareous layer. In section, aptychi consist of an outer calcareous layer; a middle layer with cellular or tubular structure; an inner calcareous layer; and, if completely preserved, a thin dark innermost layer of horny or chitinous material.
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References
Kaiser, P., and Lehmann, U., 1971. Vergleichende Studien zur Evolution des Kieferapparates rezenter und fossiler Cephalopoden, Paläont. Zeitschr., 45, 18–32.
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Lehmann, U. (1979). Aptychus, anaptychus. In: Paleontology. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-31078-9_10
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