ABSTRACT The Carboniferous genus Mitorthoceras from USA, previously assigned to orthocerids, is here restudied and assigned to the new coleoid order Tuborthocerida. Mitorthoceras has several orthocerid characteristics: longiconic shell with long body chamber; central or subcentral siphuncle containing endosiphuncular deposits; and a tubular, uni-layered connecting ring probably consisting of mixed calcareous and chitinous elements. Coleoid characteristics are: shell wall rich in a chitinous material, and lack of a nacreous layer. Characteristic for the new order is the unique structure of the outer layer of the shell wall; this layer is composed of numerous, narrow, transverse furrows, separated by distally expanding ridges and closed from the exterior by thin calcareous layers, transforming the furrows into tube-like structures. The apical end of the shell forms an acute tip. A similar tube-like furrow-layer probably occurs within the shell wall of the bactritoid-like coleoid Shimanskya that has a smooth shell surface.

中文翻译：

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具有独特壳结构的石炭纪类直角类胶体Mitorthoceras（新目Tuborthocerida、Coleoidea、Cephalopoda）；与类杆菌类胶体 Shimanskya 的比较

摘要 来自美国的石炭纪 Mitorthoceras 属，以前归于正畸目，现在重新研究并归入新的胶体目 Tuborthocerida。Mitorthoceras 有几个orthocerid 的特征：长锥壳，长体腔；中央或次中央的虹吸管含有腔内沉积物；和一个管状的单层连接环，可能由混合的钙质和几丁质元素组成。胶体的特征是：壳壁富含几丁质物质，缺乏珠光层。新秩序的特点是壳壁外层的独特结构；该层由许多狭窄的横向沟组成，由向远端扩张的脊分隔开，并由薄钙质层与外部封闭，将沟转化为管状结构。壳的顶端形成锐尖。类似的管状沟层可能出现在具有光滑壳表面的类杆菌类胶体 Shimanskya 的壳壁内。