Description. Type species Ammonites varicosus J. de C. Sowerby, 1824. According to Spath (1934) and Wright (1996), these are small ammonites (4–5 cm in diameter), evolute and serpenticone (whorls barely overlap), with a smooth stage in juveniles up to 1 cm, a subrectangular whorl section, and a low siphonal keel that usually persists until the ribbed stage. The ribs are bifurcated or alternately long and short, with umbilical tubercles and, in some forms, blunt ventrolateral ones. They vary from tall and sharp to broad and flat, but subdivisions on this basis by authors such as Scholz (1979) are not justified. Lower Cretaceous, Middle to Upper Albian. Europe, Australia, Africa, Madagascar, Iran, Pakistan, Mexico, Venezuela, Curaçao, Greenland.
A few species. Spath (1934, part 11) describes and illustrates 11 species of the genus Hysteroceras, of which the following four are frequently cited in the literature. H. binum (J. Sowerby, 1815) is compressed. Its weakened ribs on the flanks (approximately 25) give the illusion of more prominent umbilical tubercles than in the following species. H. carinatum Spath, 1922, has 36–40 ribs, weak umbilical bullae, and a keel that persists for a long time and without attenuation.
H. orbignyi (Spath, 1922) has an H/T ratio around 1 and a subquadrate whorl section. Its 30–36 ribs form an obtuse chevron as they cross the venter. H. varicosum (J. de C. Sowerby, 1824) is more or less compressed, with about 25 large, blunt ribs that cross the venter in a straight line while widening, giving tight ribs with narrow interspaces in ventral view. The keel weakens quickly and disappears over the body chamber. In 1925, Spath described another species from South Africa and Madagascar, Hysteroceras choffati, with an ovoid whorl section and ventrolateral tubercles present up to 2–3 cm in diameter. See an example on ammonites.org.
Remarks. Specimens of Hysteroceras varicosum and orbignyi from the d'Orbigny collection are also described in the revision of the "Paléontologie Française" (Fischer, 2006), and a Texan specimen of the former is described in Gale & Kennedy (2020). In the Aube region, H. carinatum, orbignyi, and varicosum have been found in the pricei and inflatum zones (Matrion, 2010). A revision would be desirable, as intermediate specimens exist between the species described in the literature, and within the same species, the different growth stages are traversed more or less rapidly. As with the genera Hoplites and Euhoplites, the genus Hysteroceras probably contains a smaller number of variable species.
| Hysteroceras (2) | orbignyi | aff. varicosum |