Campendoceras is a proterocameroceratid

Proterocameroceratidae

The Proterocameroceratidae were the first of the Endocerida. They began early in the Ordovician with Proendoceras or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor....

 genus from the Lower Ordovician of NW Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and possibly Estonia

Estonia

Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...

 that can be recognized by its slender, weakly endogastric, longiconic and annulate shell and large marginal siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 that contains endocones.

The curvature in Campendoceras is less distinct than that in Clitendoceras

Clitendoceras

Clitendoceras is a member of the Endocerida from the Lower Ordovician with an elongate shell with a slight downward, endogastric, curvature and a siphuncle that lay along the ventral margin. Common for endocerids, the chambers are short and the septa close spaced with sutures sloping forward...

or Mcqueenoceras

Mcqueenoceras

Mcqueenoceras is an early endocerid, a nautilus from the Ordovician period similar in overall form to Clitendoceras, from which it may have been derived. It lived during the later stages of the lower Ordovician. McQueenoceras, like Clitendoceras, has ventral siphuncle but the endocones are thicker...

and the siphuncle is larger.

References

• Teichert, C, 1964. Proterocameroceratidae, pp 166- 170 in the 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. Geol Soc of America and Univ of Kansas Press.