A-B. Magellania flavenscens, a modern terebratellid brachiopod: A, view of interior of dorsal valve, with all organic matter removed; B, ventral interior showing various muscle scars. C-D. Hebertella, a very common fossil brachiopod in Upper Ordovician strata: C. H. occidentalis, dorsal interior. D. H. insculpta, ventral interior
diagram and caption from Twenhofel, W. H., and Shrock, R. R., Invertebrate Paleontology, 1935. McGraw- Hill Book Co., N. Y. p.287