Cyrtorizoceras

Cyrtorizoceras minneapolis, from Sweet (1964).

Belongs within: Oncoceratidae.

Cyrtorizoceras was a genus of small cyrtoconic oncocerids from the Middle Ordovician. Species have also been assigned to this genus from the Silurian, but probably represent convergent forms (Sweet 1964).

Characters (from Sweet 1964): Compressed cyrtocones; dorsal and ventral profiles diverging constantly adorally, sides converging slightly over body chamber. Aperture with deep hyponomic sinus; surface obscurely or prominently costate. Siphuncle subventral, slender; segments slightly expanded, probably empty.

<==Cyrtorizoceras Hyatt in Zittel 1900 [=Cyrtorhizoceras (l. c.)] S64
    |--*C. minneapolis (Clarke 1897) [=Cyrtoceras minneapolis] S64
    |--C. auctidomum Foerste 1930 P68
    |--C. barbense Foerste 1936 P68
    |--C. byronense Foerste 1930 P68
    |--C. cedarvillense Foerste 1934 P68
    |--C. diagonale Foerste 1936 P68
    |--C. falciforme Foerste 1934 P68
    |--C. fosteri P68
    |--C. gasconsense Foerste 1936 P68
    |--C. hyattianum Girty 1909 P68
    `--C. longidomum Foerste 1930 P68

*Type species of generic name indicated

REFERENCES

[P68] Purnell, L. R. 1968. Catalog of the Type Specimens of Invertebrate Fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1-198.

[S64] Sweet, W. C. 1964. Nautiloidea – Oncocerida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K277-K319. The Geological Society of America and the University of Kansas Press.