Landeroceras is a straight shelled cyrtogomphoceratid from the Middle Ordovician Big Horn dolomite of Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

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The shell of Landeroceras is a subcylindrical brevicone with the siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 on one side, determined as ventral. Siphuncle segments are short and well expanded into the chambers. Connecting rings are thick, septal necks cyrochoanitic to recumbent, bullettes markedly swollen.

Landeroceras is at the end of a small offshoot within the discosorid

Discosorida

Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...

 family Cyrtogomphoceratidae that also includes Cyrtogomphoceras, and not in the line which gave rise to the Phragmoceratidae

Phragmoceratidae

The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian...

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References

• Flower, R H and Teichert, C., 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions. Mollusca, Article 6, pp 1-144.
• Teichert C. 1964. Nautiloidea -Discosorida, Treatise on Invertebrate Paleontology
  Treatise on Invertebrate Paleontology
  The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
  
  , Part K. Teichert and Moore eds.