Middle Ordovician – Late Ordovician
Beloitoceras is an extinct genus of fast-moving nektobenthic carnivores
Identification in Hand Sample:
- Strongly curved, exogastric brevicones that are ovate in cross section
- Dorsal outline of shell typically concave
- Ventral margin broadly convex
- Sutures straight and transverse adapically
- Body chamber short, laterally compressed
- Small to moderate in size (rarely up to 20 cm in length and 7.5 cm in diameter), strongly curved, exogastric brevicones that are ovate in cross section, becoming more compressed with ontogeny. Maximum shell diameter situated within the adapical one-half to two-thirds of the body chamber, shell only slightly inflated. Dorsal outline of shell typically concave for its entire length. Ventral margin broadly convex. Sutures straight and transverse adapically, sloping from the dorsum to the venter in the adoral direction with ontogeny. Cameral chambers short, cameral lengths ranging from one-fifth to one-eighth the diameter of the shell, increasing with ontogeny until onset of maturity. Body chamber short, laterally compressed; well-developed hyponomic sinus ventrally; shallow sinus dorsally. Periphract ventromyarian. Shell smooth or with fine, raised transverse ribs.
- Siphuncle small in diameter, located close to the ventral shell margin. Segments subtubular adapically, becoming expanded, elongate ovate adorally. Septal necks, adapically short, suborthochoanitic, becoming more recurved, cyrtochoanitic, with ontogeny. Connecting rings thin, homogeneous. Segments empty. Cameral deposits described for some species.
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Like Oncoceras, but less inflated and gibbous on body chamber rather than on phragmocone; dorsal profile may be concave adapically, but is straighter or slightly convex over parts of body chamber. Siphuncle small, ventral.
- The shells of this genus are exogastric compressed brevicones, the inflation of the shell relatively slight in comparison with Oncoceras, the aperture usually gently contracted, and with a hyponomic sinus. The dorsum may be concave adapically but is generally either straighter or slightly convex over parts of the living chamber. The more gibbous species appear to grade into Oncoceras, but typical forms differ from that genus in having the gibbosity when developed well on the living chamber, where both the greatest width and greatest height are attained. In Oncoceras, on the other hand, the greatest height and width are both typically found well below the base of the mature living chamber. The sutures tend to slope increasingly orad from dorsum to venter when traced orad from the beginning of the shell and develop lateral lobes. The siphuncle lies close to the venter; it is typically composed of rather slender cyrtochoanitic segments, but there is considerable variation in the form of the segments observed in the various species. A few species show segments which are scalariform, while the most typical condition is that of elongate oval segments, usually greater in length than in diameter. No organic deposits are known in the siphuncle. Vestigial cameral deposits may be present in some species appearing as faint longitudinal impressions on the internal molds of the phragmocones. In mature shells the basal zone may be developed.