Description. Genus reaching 10 cm in diameter, with a broad and shallow umbilicus. The slightly overlapping whorls increase slowly in height, with a section ranging from depressed reniform to compressed (rectangular or elliptical). The radial ribs are straight, each bearing an umbilical, a lateral, and a ventrolateral tubercle on each flank. They are opposite or alternate on either side of the venter, which bears at least as many siphonal tubercles as ventrolateral ones. In the first case, each rib crosses venter in a straight line, through a siphonal tubercle. In the second, ribs bifurcate on the ventrolateral tubercle to connect two siphonal ones, giving irregular zigzags on venter (Latil, 1994; Kennedy & Klinger, 2008a). The four tubercles per flank (including the siphonal) are already present at around 10 mm. The suture is only slightly divided, with rectangular saddles with a small median incision, and lobes that are also rectangular but narrower. Type species Ammonites lyelli d'Orbigny, 1841. This genus is found from the Lower Albian (steinmanni zone) to the beginning of the Middle Albian (benettianus zone). Distribution: Western Europe, South Africa, Madagascar, Iran, Pakistan, Mexico, Colombia, Peru, Venezuela.
Main species. According to Amédro et al. (2014), Lyelliceras vaasti Destombes, Juignet & Rioult, 1974, is the oldest and rarest representative (steinmanni zone). Its quadrangular whorl section with a flat, slightly compressed venter bears alternate ribs. A ventral rib connects the ventrolateral tubercles in a zigzag pattern and bears a small tubercle at each intersection with the siphonal line. L. pseudolyelli (Parona & Bonarelli, 1897) resembles it, but its cross-section is rounded, its siphonal tubercles are stronger, and its lateral tubercles tend to migrate upwards, becoming true internal ventrolateral tubercles (top left image). L. hirsutum from the same authors is a more robust variant, while L. huberianum (Pictet, 1847) is a variant with more widely spaced ribs and strong ventrolateral tubercles. L. lyelli (d'Orbigny, 1841), more recent, has a rounded section, opposite ribs crossing venter, and umbilical, internal and external ventrolateral, and siphonal tubercles (bottom left image, with a small malformation at mid-height). L. lyelli var. ornatissima Ciry, 1927, is a thickened form of this species.
Remarks. This genus, which descends from Tegoceras according to Amédro et al. (2014), has important stratigraphic value. Indeed, L. pseudolyelli and L. lyelli define the two subzones of the benettianus zone, with the Middle Albian beginning with the lyelli subzone. South American forms (Colombia and Peru) with two or three tubercles per flank (including the siphonal) were long considered Lyelliceras. They are now classified in the genus Prolyelliceras (see its entry).