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Oncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous), in which the connecting rings are thin and siphuncle segments are variably expanded (Flower, 1950). At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae, and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three (Sweet, 1964).
== Physical Characters==
The shells of oncocerids are primarily somewhat compressed cyrtoconic brevicones. More advanced forms include gyrocones, serpenticones, toriticones, and elongate orthocones and cyrtocones, reflective of the different families and genera (Flower, 1950; Sweet, 1964).
The siphuncle in the Oncocerida is commonly located at or near the ventral margin. Connecting rings are most commonly thin and structureless but in certain derived forms may become actinosiphonate with inwardly projecting radial lamellae. The juvenile segments in early genera are straight and tubular, with short orthochoantic septal necks inherited from the Bassleroceratidae. Later in the mature stages of early forms and throughout in the more advanced the connecting rings are inflated with cyrtochoanitic septal necks, giving what can be described as a "beaded" or "ellipsoidal" appearance (Sweet, 1964).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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