more likely possibility being for Trimeroceras to retract its protruding soft parts in its shell. Size dimorphismT. cylindricum and T. bulbosum both exhibit size dimorphism which may be linked with sex. This is largely reflected in the shells, with some specimens (macroconchs) being distinctly larger in size than the others (microconchs). In the case of T. cylindricum'', microconch shells range between 18–21 mm of height and width, while those of macroconchs are found to be in a 22–26 mm range. Though it isn't known with certainty to which sex the larger shell size belonged, it has been hypothesized that the extra space in the shell may have been taken up by an organ analogous to the spadix and antispadix of Nautilus. This would make the macroconchs representative of males. References Walter C. Sweet, 1964. Nautiloidea-Oncocerida; Treatise on Invertebrate Paleontology, Part K (K298). Geological Society of America and University of Kansas Press. Prehistoric nautiloid genera Silurian animals Silurian animals of Europe Oncocerida