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Bactritida
The Bactritida form a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (390 million years ago) and persisted until the Carnian stage of the Triassic period (235 million years ago). They are considered ancestors of the ammonoids, as well as of the coleoids (octopus, squid, cuttlefish, and the extinct belemnites).
Bactritids are distinguished from the more primitive nautiloids by the small size and globular shape of the protoconch, the so-called embryonic shell. Nautiloids have relatively large embryonic shells, and living species lay a few large eggs. In contrast, bactritids and ammonoids produced large numbers of small eggs, each housing a small embryonic shell .
== Classification and description ==
Bactritida (Erben 1964) are characterized by orthoconic to cyrtochonic shells that may be long or short with a narrow siphuncle invariably in contact with the ventral wall and sutures uniformly with V-shaped ventral lobes. Septal necks are orthochoanitic to cyrtochoanitic, the apical angle may be small or large, and the protoconch is globular to egg-shaped. The Bactritida comprise two families, the Bactritidae and the Parabactritidae.
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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