Ludvigsen, R. (1978). The trilobites Bathyurus and Eomonorachus from the Middle Ordovician of Oklahoma and their biofacies significance. Royal Ontario Museum.

Ludvigsen, Rolf. The Trilobites Bathyurus and Eomonorachus from the Middle Ordovician of Oklahoma and Their Biofacies Significance. Toronto: Royal Ontario Museum, 1978.

Ludvigsen, Rolf. The Trilobites Bathyurus and Eomonorachus from the Middle Ordovician of Oklahoma and Their Biofacies Significance. Royal Ontario Museum, 1978.