Information:  - A fish is any member of a group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered obsolete or paraphyletic by excluding the tetrapods (i.e., the amphibians, reptiles, birds and mammals which all descended from within the same ancestry). Because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.  - The nautilus (from the Latin form of the original Ancient Greek "", 'sailor') is a pelagic marine mollusc of the cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.  - Spirula spirula is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus Spirula, the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known as the ram's horn squid or the little post horn squid. Because the live animal has a light-emitting organ, it is also sometimes known as the tail-light squid.  - Permoceras , the sole member of the Permoceratidae , is a coiled nautiloid with a smooth , compressed involute shell , whorls higher than wide , earlier whorls hidden from view . The venter is rounded as are the ventral and umbilical shoulders , the flanks flattened . The siphuncle is ventrally subcentral . The suture , which is most characteristic , has a deep , narrow pointed ventral lobe and large , asymmetrical pointed lobes on either side . Permoceras is included in the nautilid superfamily , Trigonocerataceae , and is derived from the Mississippian ( L Carb ) - Triassic Grypoceratidae . Permoceras was first identified in the Lower Permian of Timur in the East Indies and named by Miller and Collinson in 1953 Permoceras almost precisely resembles Pseudonautilus from the Upper Jurassic .  - Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda, which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone. Despite their name, cuttlefish are not fish but molluscs.  - 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". In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the walls dividing the chambers.  - A cephalopod (pronounced or ) is any member of the molluscan class Cephalopoda (Greek plural , "kephalópoda"; "head-feet"). These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.    After reading the paragraphs above, we are interested in knowing the entity with which 'permoceras' exhibits the relationship of 'taxon rank'. Find the answer from the choices below.  Choices: - branch  - class  - form  - genus  - group  - order  - species  - superfamily
genus