The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral 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...

s. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras

Bassleroceras

Bassleroceras is an elongate upwardly curved, exogastric, genus with the venter on the under side more sharply rouned than the dorsum on the upper...

or possibly from some member of the Estonioceratidae

Estonioceratidae

The Estonioceratidae is a family of loosely coiled tarphycerids in which the inner side of the whorls, which forms the dorsum, is rounded or flat with no impression; and in which the siphuncle, composed of thick tubular segments, is located ventrally...

.

Tarphyceratids vary in form and siphuncle position. The siphuncle may be subcentral as in Centrotarphyceras; intermediary as in Tarphyceras

Tarphyceras

Tarphyeras is a genus of tarphyceratid with whorls rounded in cross section, having a deeply impressed dorsum and a ventral to subcentral siphuncle, known from the Lower Ord of North America...

, or subventral as in Campbelloceras

Campbelloceras

Campbelloceras: a tarphyceratid with a circular whorl section, only slightly impressed, and a siphuncle that is close to the venter in all growth stages. Differs from Tarphyceras in that the rate of expansion is greater, the siphuncle is proportionally largers, and an impression is shallower...

. Cross sections vary. Tarphyceras and Campbelloceras are rounded. Centrotarphyceras is subquadrate with broadly rounded flanks and a slightly rounded venter. Eurystomites is subquadrate and slightly compressed, with the maximum width closer to the dorsum. Pionoceras is subrounded and slightly depressed with the maximum width closer to the venter. The dorsal impression in Tarphyceras and Eurystomites is broad and deep and in Pionoceras and Centrotarphyceras, broad and shallow. In Campbelloceras and Cycloplectoceras the impression is slight, running along the dorsal median.

The Tarphyceratidae are known mainly from the Lower Ordovician (M-U Canad); gave rise to the Trocholitidae

Trocholitidae

The Trocholitidae are Tarphycerida with whorls in close contact as with the Tarphyceratidae, but in which the siphuncle, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyceratids....

 and provided the sources for the more advanced barrandeocerids

Tarphyceratid Genera

Genera in the Tarphyeratidae as named 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 (Furnish and Glenister 1964) are restricted to the miidle and upper parts of the Lower Ordovician in North America. For purposes of discussion only, they are divided into those where dorsum, which is on the inside or concave side of the volutions, is more deeply impressed and those with dorsum that is only moderately or slightly impressed.(Fig 262,K359,Furnish and Glenister, 1964)
Tarphyceratidae with dorsum deeply impressed

• Tarphyceras
  Tarphyceras
  Tarphyeras is a genus of tarphyceratid with whorls rounded in cross section, having a deeply impressed dorsum and a ventral to subcentral siphuncle, known from the Lower Ord of North America...
  
  :serpenticonic, whorls rounded vantrally and laterally, deeply impressed dorsally; rate of expansion moderate; siphuncle ventral to subcentral. Found widespread.
• Centrotarphyceras: Whorl section generally subquadrate, typically with flattened venter and flanks. Expanded more gradually than Tarphyceras. Mature siphuncle subcenral. Widespread
• Eurystomites: Large, whorls subquadrate, may be higher than wide, broadly rounded ventrally and laterally with discernable ventrolateral shoulders, dorsally impressed; siphuncle large, subcentral Widespread.
• Pionoceras : Whorl section suboval with broadly rounded venter and flanks that converge on the more narrow and impressed dorsum. Siphuncle fairly large, submarginal. Widespread

Tarphycerida with dorsum slightly to moderately impressed

• Campbelloceras
  Campbelloceras
  Campbelloceras: a tarphyceratid with a circular whorl section, only slightly impressed, and a siphuncle that is close to the venter in all growth stages. Differs from Tarphyceras in that the rate of expansion is greater, the siphuncle is proportionally largers, and an impression is shallower...
  
  : whorl section circular, slightly impressed; siphuncle close to the venter in all growth stages. Expanded more rapidly than Tarphyceras, siphuncle proportionally larger. Widespread.
• Cycloplectoceras: like Campbelloceras but with pronounced ribs. Found in Arkansas.
• Moreauoceras: like campbelloceras but more gradually expanded and with subterminal mature aperture. Found in Missouri
• Pilotoceras: small, whorls subtapezoidal to subtriangular in section; venter narrow, sides flattened and divergent, dorsum with small shallow impression; siphuncle narrow, subcentral. Widespread.

Uncertain

• Seelyoceras: Depressed, strongly annulate (transversely ribbed), curved nautiloid with ventral marginal siphuncle based on a single fragments. Affinities uncertain. Found in New York (state).