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''Dorsetensia'' is a narrowly coiled discoidal ammonite from the early Middle Jurassic, lower Bajocian, belonging to the family Sonniniidae of either the superfamily Hammatoceratoidea or Hildoceratoidea. The inner whorls are ribbed or smooth, outer whorl is smooth. The outer rim (venter) is narrow, with a keel running along the middle. The umbilicus, the opening in the middle of the shell exposing inner whorls, is of moderate size with a sharp, sometimes undercut edge. ''Dorsetensia'' is similar in general form to the genus ''Witchellia'', to the point that some have regarded ''Dorsetensia'' a subgenus of the latter. Others have regarded both ''Dorsetensia'' and ''Witchellia'' as subgenera of ''Soninnia''. The prevailing consensus is these are three distinct genera. ''Dorsetensia'' and ''Witchellia'' are separated on both morphologic and stratigraphic grounds. ''Witchellia'' is from the middle Lower Bajocian Laeviuscula subzone, Sowebvyi Zone and has a distinct tabulate venter even in smooth involute forms. ''Dorsetensia'' is from the middle Bajocian Humphriesianum Zone and has a narrow fastigate (pitched roof) venter. The inclusion of alternative superfamilies, Hildoceratoidea and Hammatoceraoidea is because to Soninniidae containing Dorsetensia is included in the Hildoderataceae in the Treatise on Invertebrate Paleontology, (Arkell, 1957) and Donovan, ''et al.'' 1981, while it is included in the Hammatoceratoidea in recent classifications, which was separated from the Hildoceratoidea by Schindewolf (1964). ==References== * Arkell, ''et al.'', 1957. Mesozoic Ammonoidea, Treatise on Invertebrate Paleontology, Part L (Ammonoidea). * The Bajocian Ammonite ''Dorsetensia'' in Skye, Scotland by N. Morton () 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dorsetensia」の詳細全文を読む スポンサード リンク
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