Ctenoceras Noetling, 1884

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 62-63

Genus Ctenoceras Noetling, 1884 , emend. Sweet (1958)

Type species: Ctenoceras schmidti Noetling, 1884 .

Diagnosis.—Slightly cyrtoconic, slightly compressed shell, with prominent sinuous annulations, growth lines, and fine longitudinal costules. Body chamber with two dorsal and one ventral impression. Siphuncle subcentral, exogastric, and nearly tubular with orthochoanitic septal necks.

Remarks.— Balashov and Zhuravleva (1962) attributed the genus to Cycloceratinae Hyatt, 1900 as a subfamily of the Orthoceratidae . Sweet (1958) attributed it with reservation to the Stereoplasmoceratidae and later ( Sweet 1964) questionably to the subfamily Orthoceratinae . The questionable status of the genus in the 1960s had its reasons in the unknown internal characters. New material from Estonia shows hat the connecting ring of Ctenoceras schmidti resembles closely that of Orthoceras regulare . The Fig. 11A shows a spherulitic−prismatic outer connecting ring as it is well known in Orthoceras regulare and Orthoceras scabridum . It must be summarised that Ctenoceras shares a lot of characters with typical members of the genus Orthoceras : an ornamentation consisting of growth lines, which are rectangular crossed by longitudinal costules, a characteristic structure of the connecting ring, shape of septal necks and typical impressions of the body chamber. Ctenoceras is therefore considered to be a close relative of Orthoceras .

Siphuncle subcentral. Siphuncular diameter about 0.15 mm of the respective diameter of the conch. Hypo−, episeptal, and endosiphuncular deposits not observed.

Living chamber: Conical adult living chamber with two crescent shaped, lateral impressions and a narrow, deep ventral impression at the last third of the living chamber. Length of the living chamber approx. 60 mm. Aperture widens slightly adorally to the constriction.

Species included.— Ctenoceras schmidti Noetling, 1884 , Ctenoceras chinense Lai and Wang, 1981 .

Figures

Fig. 11. Polished sections of the siphuncle. A. Ctenoceras schmidti (Noetling), NRM−Mo 160668b, Kandel (Estonia). Note the calcified connecting ring and the thin lining of a sphaerolitic−prismatic layer at the outer surface of the connecting ring, × 25. B. Archigeisonoceras folkeslundense sp. nov., NRM−Mo 3223, Källa (Öland). Note the porous structure of the calcified connecting ring and the sphaerolitic−prismatic layer, × 10. C. Orthoceras scabridum Angelin, NRM−Mo 154771, Södre Bäck (Öland), × 30. D. Plagiostomoceras fragile sp. nov., NRM−Mo 160559, Gardslösa (Öland), × 6.3. E. Archigeisonoceras repplingense sp. nov., NRM−Mo 3285 not numbered, Hälludden (Öland), note the asymmetrical development of the endosiphuncular annuli, × 18. F. Archigeisonoceras repplingense, NRM−Mo 3284, Hälludden (Öland), apical part, × 18. G. Archigeisonoceras repplingense sp. nov., NRM−Mo 3293, Torp (Öland), note the thin sphaerolithic−prismatic layer of the connecting ring, × 22.