Orthoceras Brugière, 1789

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 59

Genus Orthoceras Brugière, 1789

Type species: Orthoceratites regularis Schlotheim, 1820 .

Diagnosis.— “Straight orthoconic shells with longitudinal impressions of the living chamber. Exterior sculptured with transverse lines of growth forming a banding somewhat similar to that in Geisonoceras , but the bands are composed of densely crowded minute longitudinal ribs, which are especially well shown on a slightly weathered surface. Apertural angle small. Air chambers and siphuncle of medium size; siphuncle central or subcentral.” ( Troedsson 1931 a: 12).

Remarks.—Beside the many definitions of the genus Orthoceras (e.g., d'Orbigny 1849; Balashov and Zhuravleva 1962; Sweet 1964; Dzik 1984) the diagnosis given by Troedsson (1931 a) matches best the observations described herein. Troedsson’s definition of the number of longitudinal impressions is not too narrow to include the strong variations of these features (see e.g., Orthoceras quinquefeovatum Noetling, 1884 , which is shown by Troedsson (1931 a) to be a variety of Orthoceras regulare with very long impressions) and it gives a certain importance to the shell ornamentation. The sculpture of O. regulare , O. bifeovatum , and O. scabridum is unique among the Baltoscandic orthoceratids.

Species included.— Orthoceras regulare Schlotheim, 1820 , Orthoceras bifeovatum Noetling, 1884 , Orthoceras scabridum Angelin, 1890 .