Isorthoceras undefined-C

Isorthoceras sp. C Fig. 12F Material examined ESTONIA • 1 spec.; Vormsi Island, Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-49. Description This specimen is a fragment of a body chamber and four chambers of the phragmocone with a nearly tubular circular conch with a diameter of 9.2–9.5 mm and an angle of expansion of ca 1°. The preserved part of the body chamber is 16 mm long, the septa are ca 1.8 mm apart, respectively (RCL = 0.19), and the septal foramen is eccentric with an RSP of 0.28. The septa are directly transverse. The conch surface is not preserved. Remarks This specimen can be compared with Isorthoceras sp. B, but its lower angle of expansion at smaller conch diameters indicates an even smaller adult size (ca 10 mm) of this species. The lack of external shell prohibits any species level determination. However, based on the small adult size, conch shape and siphuncular position the species can be related to I. junceum. Another small species known from Late Ordovician strata of Sweden, I. curvilineatum Kröger, 2013, differs in having a central siphuncle.