Kochiella Poulsen, 1927, and Hadrocephalites new genus (Trilobita: Ptychopariida) from the early Middle Cambrian of western North America

Kochaspids are an informal group of ptychopariid trilobites that were both abundant and widespread in the early Middle Cambrian of North America. Based on the reassociation of pygidia and cranidia of some kochaspids, Kochiella Poulsen, 1927, is redefined and Hadrocephalites n. gen. is proposed. Hadrocephalites includes taxa previously assigned by Rasetti and Palmer to Schistometopus Resser, 1938a. Schistometopus is considered nomen dubium. Representatives of Kochiella and Hadrocephalites from the Pioche Shale and Carrara Formation of Nevada are described, including the new species Kochiella rasettii, K. brevaspis, Hadrocephalites lyndonensis, and H. rhytidodes. Other kochaspids previously assigned to Kochaspis Resser, 1935; Eiffelaspis Chang, 1963; Schistometopus; and Kochiella are discussed and some are reassigned. The type specimens of Kochiella augusta (Walcott, 1886); K. crito (Walcott, 1917b); K. chares (Walcott, 1917a); K. mansfieldi Resser, 1939; K. arenosa Resser, 1939; Hadrocephalites carina (Walcott, 1917b), and H. cecinna (Walcott, 1917b) are re-illustrated.