Wikipedia
The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras or possibly from some member of the Estonioceratidae.
Tarphyceratids vary in form and siphuncle position. The siphuncle may be subcentral as in Centrotarphyceras; intermediary as in Tarphyceras, or subventral as in Campbelloceras. 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 and provided the sources for the more advanced barrandeocerids