Curtoceras Ulrich, Foerste, Miller & Furnish 1942

Genus Curtoceras Ulrich, Foerste, Miller & Furnish, 1942 Type species Lituites Eatoni Whitfield, 1886; by original designation. Diagnosis Genus of the family Trocholitidae with slowly expanding (WER decreasing up to ~ 1.80), subevolute to evolute conch (UWI increasing up to ~ 0.55), discoidal to extremely discoidal in shape (CWI decreasing up to ~ 0.30), with some part of the last half a whorl widely uncoiled. Whorl profile weakly compressed, equidimensional or weakly depressed (WWI ~ 0.90–1.10); whorls weakly embracing or not embracing, rounded or slightly flattened across venter and / or flanks. Siphuncle ventral in initial chamber, subdorsal or subcentral after one volution, septal necks short (<0.20 of chamber height), orthochoanitic. Shell surface smooth or with raised lirae, low ribs or annuli that form ventral sinus. Suture line nearly straight or with shallow lateral and / or ventral lobe (Ulrich et al. 1942; Furnish & Glenister 1964; Kröger & Aubrechtová 2018). Species included North America (Billings 1862; Whitfield 1886; Foerste 1938): Lituites Eatoni Whitfield, 1886; Lituites internastriatus Whitfield, 1886;? Schroederoceras minganense Foerste in Twenhofel, 1938;? Lituites palinurus Billings, 1862. Great Britain (Blake 1882): Nautilus (Trocholites) scoticus Blake, 1882. Northern Europe and Russia (Eichwald 1840; Lossen 1860; Schröder 1891; Hyatt 1894; Balashov 1953; Kröger & Aubrechtová 2018): Curtoceras abditus Kröger & Aubrechtová, 2018; Curtoceras kerstovense Balashov, 1953; Nautilus teres Eichwald, 1840; Curtoceras meyeri sp. nov.; Trocholites macromphalus Schröder, 1891. Argentina (Aceñolaza et al. 1977): Curtoceras kayseri Aceñolaza, Durand & Taddei, 1977. Remarks The morphology of Curtoceras is similar to that of Trocholites. The species of Trocholites are less discoidal or even pachyconic in conch shape with more depressed and more impressed whorl profile; the body chamber is not uncoiled in Trocholites and very often, the terminal aperture bears conspicuous flares. However, a clear separation of the two genera is difficult and possibly subjective; species with a transitional morphology or for which the fully adult growth stage is unknown are difficult to assign unequivocally to one of the two genera. Discoceras differs from Curtoceras in the much larger adult size (over 100 mm) and the presence of frilled, imbricated ornament elements. The whorl profile in Discoceras is often more strongly compressed than in Curtoceras, and subrectangular, subquadratic or trapezoidal in shape. However, separation of Curtoceras and Discoceras may be difficult in incomplete specimens or specimens representing juvenile or immature growth stages. Geographic and stratigraphic occurrence North America, Argentina, Great Britain, Estonia, Sweden (Island of Öland), St Petersburg and Kaliningrad region of Russia, northern Germany, northern Poland,?Tibet; Ordovician.