''Turnoceras'' is a genus of

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s belonging to the oncocerid family Nothoceratidae. Its shell is broadly expanding and exogastrically curved such that the flattened dorsum is on longitudinally concave side.

Aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

unconstricted with no hyponomic sinus for the water jet funnel.

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

along the outer, ventral, side, with radial, plate-like actinosiphonate deposits occupying the interior. '' Conostichoceras'' is similar externally, but has a

with a trapezoidal aspect to its siphuncle segments. '' Perimecoceras'' has similar concave siphuncle segments but is tubular in form.

References

* Sweet, W.C. 1964; Nautiloidea -Oncocerida;

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

, Part K ; Geological Society of America and University of Kansas press; Teichert and Moore (eds).
Turnoceras-Paleodb

Sepkoski Cephalopod Genera
Prehistoric nautiloid genera Oncocerida {{paleo-Nautiloidea-stub