Middle Ordovician – Late Ordovician
Lambeoceras is an extinct genus of nektobenthic carnivores
Identification in Hand Sample:
- Large to very large shells
- Sutures form broadly rounded lobes, generally very deep
- Shell straight, very strongly depressed
- Segments have prolonged constricted part where septa bend backward
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Suttner and Kroger (2006):
- Large orthocones with strongly compressed, elliptical cross-section. Both sides of cross-section evenly and shallowly convex. Sutures with broad sinus across ventral and dorsal sides and acute lateral saddles. Siphuncle slightly removed from conch margin. Broadly expanded siphuncular segments with strongly recurved septal necks pointing forward and outward.
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Large to very large shells, both flat sides evenly and gently convex; sutures with broad sinus across ventral and dorsal sides and acute lateral saddles. Siphuncle subventral; segments either like Gonioceras or with more prolonged constricted part where septa bend backward behind septal foramen; brims strongly recurved, pointing forward and outward; endosiphuncular canal system simple, known from all species except type-species. (Genus originally proposed as subgenus of Tripteroceras Hyatt, 1884)
- Shell straight, very strongly depressed, with the dorsum and venter somewhat convex but strongly separated by sharp lateral angles. The sutures form broad rounded lobes, generally very deep, over dorsum and venter separated by angular lateral saddles. Generally the suture describes a lobe on the venter somewhat more strongly curved and narrower than that of the dorsum. The sutures show only a very slight reversal of curvature toward the sides, not present in all species. The septa are strongly curved laterally but nearly flat vertically. The siphuncle is located ventrad of the center of the shell, is large, and is typical of the Actinoceroidea in its construction and deposits. Owing to the extreme flatness of the shell the siphuncle differs markedly in appearance depending upon the direction of the section from which it is studied. A horizontal sections shows broadly expanded segments. The septa appear to approach each other where they make up part of the siphuncle and are adnate to the next adoral connecting ring but are turned slightly farther apicad than is normal. The true brim cannot usually be seen in opaque sections as it is very short and is usually complicated by the development of the circulus; the whole being frequently obscured by recrystallization. The true structure has, however, been demonstrated by Teichert. The connecting ring is nearly free at its adoral end but broadly adnate adapically. In vertical section the septa bend abruptly apicad and appear to be orthochoanitic. They do, however, have short recumbent septal necks. The connecting ring expands rapidly, forming an inflated segment and is broadly adnate to the next septum at its adapical end. The structure is faintly suggestive of that of Huronia. The usually annulosiphonate deposits are developed with radial and central canals remaining.
- The surface features (Leith, 1942) are known for only the genotype. They consist of transverse bands which form a slight crest on the dorsum and a low broad sinus over the entire venter. Shells of this genus may attain immense size, rivaling the larger endoceroids.