5-507: Tarphyceras 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 (U Canad ) of North America. It differs from Campbelloceras in that Campbelloceras is only slightly impressed, from Centrotarphyceras in that Centrotarphyceras is subquadrate and has a central siphuncle, and from Trocholites in that although Trocholites
10-703: 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
15-425: Is subcircular in cross section, the siphuncle is subdorsal. Taphyceras may be derived from Campbelloceras through development of tighter coiling and a deeper impression along with a smaller siphuncle that may be further offset from the venter. Tarphyceras was named by Hyatt in 1894. Tarphyceratidae The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles . The dorsum
20-556: 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
25-626: 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 Genera in the Tarphyeratidae as named in the Treatise on Invertebrate Paleontology Part K (Furnish and Glenister 1964) are restricted to the middle and upper parts of
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