In the paired ventral part of the fin line, normally only the pectoral and pelvic fins survive (but the Devonian acanthodian fish Mesacanthus developed a third pair of paired fins); but along the non-paired parts of the fin line, other fins develop. In tetrapods, only the four paired fins normally persisted, and became the four legs. Notomelia and cephalomelia are atavistic reappearances of dorsal fins. Some other cases of polymelia are extra development along the paired part of the fin lines, or along the ventral posterior non-paired part of the fin line. Notable cases
Humans
1995: Somali baby girl born with three right arms. In the summer of 2005, a baby girl named Destiny was born with a fully formed extra leg in Detroit. This was the result of a conjoined twin scenario. In March 2006, a baby boy identified only as Jie-jie was born in Shanghai with a fully formed third arm: he had two full-sized left arms, one ventral to the other. This is the only documented case of a child born with a fully formed supernumerary arm. It is an example of an extra limb on a normal body axis.
He believes that he has been invited to be interviewed on television because he is a nice person, and is dismayed to learn that he has only been invited because the interviewer is curious about his unusual condition. The Dark Backward is a 1991 comedy film directed and written by Adam Rifkin, which features Judd Nelson as an unfunny garbage man who pursues a stand-up comedy career. When the "comedian" grows a third arm out of his back, he becomes an overnight hit. Justice, the main antagonist in the cartoon Afro Samurai, has a fully formed extra arm protruding from his right shoulder. Paco, a playable character in Brawlout, is a four-armed frog- and shark-like creature. Spiral, character in the X-Men stories of Marvel Comics, has six arms. A 6-armed Spider-Man frequently appears as an alternate reality incarnation of the character, and is sometimes referred to as "Polymelian". In the Mortal Kombat series, the Shokan is a race of humanoids known to have four arms. Ibid: A Life is a fictional biography of Jonathan Blashette, a man with three legs. Zaphod Beeblebrox, a character in Douglas Adams Hitchhikers Guide to the Galaxy, has a third arm as well as a second head. See also
Dipygus
Dysmelia
Polydactyly
Polysyndactyly
Supernumerary body parts

References
Sources
Avian Diseases, 1985 Jan-Mar; 29(1): pp. 244-5. Polymelia in a broiler chicken., Anderson WI, Langheinrich KA, McCaskey PC. : "A polymelus monster was observed in a 7-week-old slaughterhouse chicken. The supernumerary limbs were smaller than the normal appendages but contained an equal number of digits.".