Gorbyoceras stumburi

Gorbyoceras stumburi (Kröger, 2013) Figs 12J, 18 Dawsonoceras stumburi Kröger, 2013: 32–34, figs 16e, 17b. Dawsonoceras sp. – Kröger & Isakar 2006: 157, figs 10e, j, 12h. Diagnosis Annulated orthocones with angle of expansion of ca 5° and circular cross section; ca five relatively shallow annulations occur in a distance similar to conch diameter; approximately 40 longitudinal lirae occur around shell circumference, more than 10 pronounced transverse striae occur per cycle of annulations; septal necks achoanitic; siphuncle subcentral, fusiform, expanded within chambers. (Adopted from Kröger 2013.) Material examined ESTONIA • 1 spec.; Vormsi Island, Hosholm shore; Adila Formation, Pirgu Regional Stage; TUG 1745- 47 • 1 spec.; Harilaid; Vormsi – Pirgu regional stages; TUG 939-30 • 1 spec.; Vormsi Island, Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; TUG 939-21. Type locality and horizon Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician. Description Specimen TUG 1745-47 is a slightly curved fragment of a mature body chamber with a length of ca 35 mm and a maximum diameter of 20 mm. Approximately five annulations occur in a distance similar to the conch diameter. The surface is ornamented with ca 40 longitudinal lirae around the circumference and fine growth lines. At its base, the fragment has a diameter of 19 mm and a chamber of the phragmocone is preserved with two septa which are 5 mm distant. The septa are located in the troughs of the respective annulations and are directly transverse, parallel to the annulations. The septal foramen is poorly preserved but apparently slightly eccentric in position and located toward the convex side of the conch curvature. Specimen TUG 939-30 (Fig. 12J) is a nearly tubular fragment of a body chamber with relatively well-preserved conch surface showing ca 40 longitudinal lirae across the circumference and fine transverse lirae or raised growth lines (ca 10–12 per cycle of annulation). The spacing of the annulation is relatively narrow with 5–6 cycles of annulations per distance similar to the corresponding conch cross section. Remarks No new data on the internal characters of this species can be added with the Estonian material. The species was originally placed within Dawsonoceras. However, based on the similarity of the known characters from this species with other species of Gorbyoceras and following the more restricted genus diagnosis of Dawsonoceras of Horny (1956), followed herein (see above), a placement within Gorbyoceras is suggested, herein.