Turnoceras is Devonian cephalopod belonging to the oncocerid

Oncocerida

The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...

 family Nothoceratidae. Its shell is a broadly expanding exogastric cyrtocone with a flattened dorsum -on the inner concave side, unconstricted aperture, no hyponimic sinus, and ventral siphuncle -on the outer, convex side- composed of concave segments with actinosiphonate structures occupying the interior.

Conostichoceras

Conostichoceras

Conostichoceras is an exogastric, breviconic oncocerid included in the family Nothoceratidae, known from the Middle Devonian of central Europe and Upper Devonian of Australia...

is similar externally, but has a siphuncle with a trapezoidal aspect to its siphuncle segments. Perimecoceras

Perimecoceras

Perimecoceras is a nothoceratid, nautilitoid order oncocerida, with a slowly expanding, compressed, cyrtoconic shell, known from the Upper Silurian of central Europe....

has similar concave siphuncle segments but is tubular in form.

-References-

• Sweet, W.C. 1964; Nautiloidea -Oncocerida; Treatise on Invertebrate Paleontology
  Treatise on Invertebrate Paleontology
  The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
  
  , Part K ; Geological Society of America and University of Kansas press; Teichert and Moore (eds).
• Turnoceras-Paleodb
• Sepkoski Cephalopod Genera