直角石
直角石（学名Orthoceras）是一属已灭绝的鹦鹉螺类。其化石非常普遍，分布在世界各地的海洋地层，尤其是石灰岩。
直角石的壳幼长，中央是住室，其次是连室细管。壳表面有很多幼细的细褶襞。多种其他相似的物种都被分类在米契林角石属中。虽然直角石的外壳像鹦鹉螺，但却应该更为接近蛸亚纲。
Orthoceras
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Orthoceras 直角石
Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros (Sweet 1964:K222).
Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially in limestone.
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