Concretions containing many, closely packed fossils are sufficiently numerous to warrant special attention and probably a distinctive designation. A few examples emphasize the problem of origin: (a) one concretion from shale above the no. 5 coal bed of Illinois (Pennsylvanian) yielded at least 1,000 uncrushed Composita argentea; the sparse fauna of the matrix is extensively crushed; (b) 3 concretions from the Deseret Limestone (Mississippian) of north-central Utah yielded several hundred specimens of the ammonoid Dzhaprakoceras known previously only from Asia; (c) one concretion from the Colorado shale (Early Cretaceous) near Harlowton, Montana, yielded 1,400 uncrushed specimens of the ammonite Gastroplites, a relatively rare genus in North America, (d) numerous concretio s from the Ferron Sandstone (early Late Cretaceous), Emery County, Utah, under investigation by the writers, have a well-preserved molluscan fauna, whereas the matrix is practically unfossiliferous; and (e) very abundant concretions in the Fox Hills Formation (late Late Cretaceous) of South Dakota commonly show dense accumulations of several molluscan species; fossils are otherwise sparse.

It is proposed that such fossiliferous concretions have resulted from selective, purposeful, food-gathering or hoarding activities of large vertebrate or invertebrate marine animals. No other agency seems capable of bringing together large numbers of sessile, somewhat heavy, and probably still-living animals into small compact masses. Whether these represent food not yet ingested, undigestable residues (coprolites?), or contents of some part of the alimentary canal in process of digestion is not known. The term "gastric concretion" is proposed as sufficiently broad and descriptive to cover this type of accumulation.

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