Description. Type Hamites raulinianum d'Orbigny, 1842. Heteromorph ammonites coiled in open spiral, in one plane or slightly helicoid, with a terminal hook and several more or less straight shafts. Ribs rectiradiate, weak or absent on the dorsum (i.e., the inner part of the shell's curvature), with ventrolateral and in some species midlateral tubercles on some or all ribs. Ribs joining tubercles across venter may be flattened but never doubled (fibulate). On the test, the tubercles correspond to spines. Simple suture with 3 bifid saddles and 4 lobes, including a small, trifid second lateral lobe L2 (Spath, 1939; Casey, 1961; Wright, 1996). Upper Aptian?, Lower and Middle Albian. Europe, Madagascar, India, and Peru.
Subgenera and some species. Protanisoceras sensu stricto from Lower Albian is entirely coiled in a plane, examples P. (P.) raulinianum (d'Orbigny, 1842), P. (P.) cantianum Spath, 1939, and P. (P.) blancheti (Pictet & Campiche, 1861, top image). Heteroclinus Spath, 1961, from Middle Albian, has a compressed section, a helicoid initial spiral, and a larger second lateral lobe, like P. (H.) nodosum (J. Sowerby, 1818) and P. (H.) flexuosum (d'Orbigny, 1842, bottom image). Torquistylus Casey, 1961, has blunt, closely spaced ribs that cross the siphonal line obliquely. The curvature of its cross (body chamber) deviates from the coiling plane. It contains a single species, P. (T.) anglicum (Spath, 1929), from Lower Albian.
Remarks. Klein (2015) listed 36 species. Many are doubtful because they are very similar and known only in fragments. The exact form of complete adults is also unknown for most taxa. As with the Hoplites, specimens intermediate between two or even three published "species" are often found on the field: a statistical analysis would likely reveal a small number of highly variable species. For the beginner, Protanisoceras can be confused with Aptian genera with ventral and lateral tubercles, such as Lithancyclus, Toxoceratoides, or Tonohamites: it is therefore essential to know the ammonite zone of a specimen.
In the Albian of the Aube region, the genus extends from the floridum zone to the dentatus zone. Three other genera with comparable forms are also found. Hamites, with untuberculate ribs, occurs throughout the Albian but is rare in Lower Albian. Anisoceras, with 2 or 4 tubercles per rib, has fibulate ribs between the lateral ones. It is typical of Upper Albian, but one species, Anisoceras arrogans (Giebel, 1852), occurs from the subhilli zone (Lower Albian) to the intermedius zone (Middle Albian). Idiohamites has a compressed elliptical section and only ventrolateral tubercles; it is found only in the Upper Albian.