| Izvleček: | Calcareous concretions from middle and upper Cenomanian (Cretaceous) shale sequences in Montana and Wyoming yield, on rare occasions, abundant minute ammonites. Some are juveniles of large species, and give valuable evidence on early ontogeny and evolutionary affinities of these taxa. They provide evidence for the probable evolutionary origins of a series of hitherto undescribed progenic dwarf genera, adult at 4.5-16.5 mm diameter that are a remarkable feature of these assemblages. Five new genera, Kastanoceras, Alzadites, Microsulcatoceras, Cryptometoicoceras and Buccinammonites are introduced. Some 33 species are described from Montana, Wyoming, Utah, New Mexico and Texas. -from Authors
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