The structure of the shell wall in the Ordovician Orthoceras differs from that in the extant Nautilus. It consists of two equal thick layers: an outer probably nacreous layer and an inner prismatic...

中文翻译：

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Baltoscandia（头足类：orthoceratida，calciosiphonata）奥陶纪 Orthoceras 壳壁中迄今为止未检测到的孔管网络

奥陶纪正角类的壳壁结构与现存的鹦鹉螺不同。它由两个相同厚度的层组成：外部可能是珠光层和内部棱柱形......