Plagiostomoceras Teichert and Glenister, 1952

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 64

Genus Plagiostomoceras Teichert and Glenister, 1952 Type species: Orthoceras pleurotomum Barrande, 1866 .

Diagnosis.— Michelinoceratinae with a long, slender conch with a slightly compressed cross section. Aperture strongly oblique with a prominent lobe at its antisiphonal side. Surface of the shell covered with weak growth lines.

Remarks.— The genus Plagiostomoceras is very similar to Protobactrites Hyatt, 1900 . Differences between the two genera may be the more eccentric siphuncle and the smooth shell in Protobactrites . Because no information about the internal characters are given for the latter, it remains a dubious taxon. Therefore, the use of its possible junior synonym ( Plagiostomoceras ) is preferred here. The early growth stages of the type of both Protobactrites and Plagiostomoceras are unknown. Dzik (1984) placed in the genus two embryonic specimens from the Devonian of the Holy Cross Mountains, which show a globular first chamber. However, by definition of Ristedt (1968) the Michelinoceratinae , in which the genus is grouped herein, are characterised by a shell which did not develop a constriction between the initial chamber and the rest of the conch. Thus, the genus could not be placed in the Michelinoceratinae . The decision of whether or not Plagiostomoceras could be placed into the Michelinoceratina must wait until the type species is better known. The two species of Plagiostomoceras of the Baltoscandic Orthoceratitic Limestone are the oldest known members of the genus.