Arionoceras arion (Barrande 1868), A. affine (Meneghini 1857), A. canonicum (Meneghini 1857), A. densiseptum Kobayashi 1983, A. submoniliforme (Meneghini 1857) = (Psilorthoceras procerum Ristedt 1968, P. crassum Ristedt 1968, P. subcrassum Ristedt 1968), A. capillosum (Barrande 1868), A. repetitum (Barrande 1870) A. septentrionale Zhuravleva 1978, A. severum (Barrande 1866), A. simplex (Desnoy in Meneghini 1857, A. canonicum (Meneghini) synonymous with A. affine) and A. valens (Barrande 1868) are considered to belong in the family Arionoceratidae Dzik 1984. The genera Caliceras Kolebaba and Psilorthoceras Ristedt represent synonyms of Arionoceras Barskov. Small, but detectable differences in the protoconch dimensions, angle of expansion, and a diverse septal spacing between Meneghini's 1857 species A. affine and A. submoniliforme could, perhaps, be interpreted as sexually dimorphic characters of A. submoniliforme. If the morphological variation in this ancient nautiloid species is interpreted in light of patterns present in modern Nautilus (the sexual dimorphism of which is due to the broader aperture of the males than that of the females) we must consider as the male's features those of A. submoniliforme and those of A. affine as the female (see also Stenzel 1964, Tanabe and Tsukahara 1987, Saunders & Ward 1987, and Hayasaka et al. (1987). Stridsberg (1988) also argued for size variation as indicator of sexual dimorphism in the trimeroceratid Torquatoceras undulatum Stridsberg 1988, though this conclusion is currently accepted as a working hypothesis only.
The genus Akrosphaerorthoceras Ristedt 1868 bears a very little protoconch, which is a
characteristic of the Family Arionoceratidae. The genus Temperoceras
Barskov 1960 lacks recent studies of its apex;
especially as concerns its protoconch. The arguments considered
by Dzik regarding placement of the genus Parakionoceras Foerste 1928 mainly concern features of its
suborthochoanitic siphuncle. On the other hand Foerste's genus
seems sufficiently peculiar in characters relating to its outer
ornamentation-structure to isolate Parakionoceras from the
other genera. It is the opinion of the writer that the typical
features of arionoceratids, besides those reported by Dzik,
primarily consist of the suborthochoanitic, terminally very
acuminate, and short septal necks, connecting rings slightly
expanding within chambers and a big protoconch usually more than
two mm in length, ogive-like in shape crossed by long caecum
linked apicad to the beginning of the conch by a short prosiphon
(see Serpagli & Gnoli 1977, p. 183, text-figs. 10, 11, pl. 6, figs.
2a-7, 9a-b, pl. 8, figs. 1-3, and here Fig. 1)
. As far as O. affine Meneghini 1857
(previously O. affine Portlock 1843), is concerned, as reported in the
Engeser’s Internet page, it must be noted that Histon (1997) and Histon & Sevastopulo
(1993) taxonomically
revised this Irish Lower Carboniferous species as Mitorthoceras
affine (Portlock).
To conclude, it is the opinion of this writer that at present the family Arionoceratidae Dzik 1984 embraces only the genus Arionoceras Barskov. Additional work in this area may include (1) further taxonomic study to increase the number of diagnostic features attributible to this genus and (2) to consider Arionoceras Barskov as belonging in Zhuravleva' s Family Geisonoceratidae as already proposed by Serpagli & Gnoli (1977) or Gnoli (1990, 1998, in press). Finally, it is worth noting that this writer has recently had the opportunity to examine representatives of the Arionoceratidae in all the terranes or microplates [at least in: Tinduf Basin, Anti-Atlas, Mid-North Armorican Domain (bona fide Babin et al., 1979), Montagne Noire, SW Sardinia, Carnic Alps and Central Bohemia (Paris & Robardet, 1990: Robardet et al, 1994 and Gnoli in preparation)] that together constitute the northern belt of Gondwana during Silurian time.