Arionoceras Barskov, 1966

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 72

Genus Arionoceras Barskov, 1966

Type species: Orthoceras arion Barrande, 1868 .

Diagnosis.—The orthoceraconic or slightly cyrtoceraconic Arionoceras shows a relatively high apical angle. The cross section of the shell is circular. The surface of the shell is smooth or shows a weak transverse sculpture. The siphuncle is central. The septal necks are suborthochoanitic, very short, and acuminate. Their length is less than the diameter of the septal foramen. The connecting ring is cylindrical. The protoconch is large, pointed apically, ogival in shape, and bent ventrally. Cameral deposits developed in the apical chambers of adult specimens (slightly modified from Serpagli and Gnoli 1977).

Remarks.— A definitive placing of a specimen in the genus is possible only if the protoconch is known. But the specific shape of the nearly adult conch and siphuncular tube is considered as a valuable criterion for determination. The Ordovician Arionoceras differs significantly from all other known Baltic cephalopods in even these specific features of the nearly mature shell. As pointed out by Gnoli (1998), the genera Caliceras Kolebaba and Psilorthoceras Ristedt represent synonyms of Arionoceras Barskov. Regarding the type species of the Arionoceratidae it must be mentioned that Orthoceras affine Meneghini, 1857 is a senior synonym of Orthoceras arion Barrande, 1868 (see Serpagli and Gnoli 1977). The name Orthoceras affine , however, was preoccupied by O. affine Portlock, 1843 which is a Mithortoceras ( Histon, 1998). Consequently, O. arion Barrande, 1868 is the only valid name of the type species of the Arionoceratidae .