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A primary difference between ammonites and nautiloids is the siphuncle of ammonites (excepting Clymeniina) runs along the ventral periphery of the septa and camerae (i.e., the inner surface of the outer axis of the shell), while the siphuncle of nautiloids runs more or less through the center of the septa and camerae.
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Most of the generic names in this list come from Jack Sepkoski's 2002 compendium of marine fossil genera, which can be corroborated by other sources such as Part L, Ammonoidea, in the Treatise on Invertebrate Paleontology. Additional generic names included come from the Treatise or various peer review scientific journals.
Goniatite shells are small to medium in size, almost always less than 15 centimeters (5.9 inches) in diameter and often smaller than 5 centimeters (2.0 inches) in diameter. The shell is always planispirally coiled, unlike those of Mesozoic ammonites in which some are trochoidal and even aberrant (called heteromorphs).
Fontannesia is an ammonoid cephalopod genus with a small to medium size, evolute, discoidal shell that was extant during the Jurassic Period.The sides are ribbed, the venter has a single median keel, and tubercles are lacking.
Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post-Triassic ammonites.
Phricodoceratidae is a family in the Eoderoceratoidea, aberrant ammonites from the Lower Jurassic characterized by a large adult size and a marked change of shell form and ornament with growth. Shells are stoutly ribbed, early growth stage is round-whorled with spines, followed by a high-whorled late growth stage with smooth, modified ribbing.