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Recently I went fossil hunting along a road cut revealing some of the Coburn Formation, latest Ordovician. I was stunned to find that so much of the ecosystem was made up of only Trilobites and Cephalopods. I found trilobites such as Isotelus and Cryptolithus (First picture). My find of the day was a large, very heavy plate of rock that preserves different parts of large Isotelus gigas from multiple individuals, and the circular cross-section of a small cephalopod (Second picture). However, I'm having trouble identifying these cephalopods. In the very few that preserve the outer sell, faint striae can be observed. The septa, when visible, are close together. On some of the smaller specimens, which may be a different species or the same, the uncrushed cross-section reveals what appears to be a small, eccentric siphuncle (Sixth picture). Most specimens, because of their size, are crushed flat. Only much smaller individuals sometimes retain their original shape. Arrows indicate septa unless stated otherwise. The large cephalopods appear similar to both "Michelinoceras" and Geisonoceras tenuistriatum, but this species is restricted to the Whitby formation in Ontario. Could anyone help?