Homeomorphy of brachiopod shells

Simulation of one shell morphotype by another constitutes homeomorphy, and resemblances of this sort are quite numerous among brachiopods. The near identity in appearance can be very misleading, and it is necessary to take into account shell structure and internal features. Homeomorphs often not only belong to different genera, but they maybe classed in different families and even (as the following examples show) different orders. Some lived at the same time, others were separated by many millions of years.

1a, b, Productorthis eminens (Pander), brachial view and median longitudinal section showing shell form, Middle Ordovician, Russia. Width 1.5 cm. This impunctate shell belongs to the superfamily Orthacea and order Orthida.
2a, b, Dictyodostus americanus Dunbar & Condra, brachial view and median section, Late Pennsylvanian, Kansas, width 6 cm. This is a pseudopunctate shell belonging in the family Productid (order Strophomenida)
3a, b, Tetractinella trigonella (Schlotheim), brachial and posterior views, length and width 1.8 cm. An impunctate spire-bearing brachiopod (order Spiriferida); Middle Triassic, Italy.
4a, b, Cheirothyris fleuriausa (d'Orbigny), brachial and posterior views, width 2 cm; a punctate loop-bearing shell of the order Terebratulida. Late Jurassic, Germany.

from Moore, Lalicker and Fischer, Invertebrate Fossils, McGraw-Hill Book Company Inc, 1952, p.218

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