Ellesmeroceras is the type genus for the Ellesmeroceratidae, a family of primitive nautiloid cephalopods, that is characterized by its small, generally compressed, gradually expanded, orthoconic shell, found in Lower Ordovician marine sediments. The septa are close spaced and the siphuncle is ventral, about 0.2 the diameter of the shell. Septal necks are typically (short, straight) but may slant inwardly or reach halfway to the previous septum. Connecting rings are thick. As common for the Ellesmerocerida, Ellesmeroceras has diaphragms within the siphuncle tube.