Well I just found new fodder for my nightmares. The two-headed fetus was removed from a pregnant female captured in the Gulf of Mexico near Key West, Florida, U.S.A. by a commercial fishing vessel (F/V Island Girl) on 7 April 2011. According to the authors of the recent study describing this anomaly.
Each head has five pairs of gills and gill openings, a single pair of eyes, a single pair of nares and a mouth with well-developed dentition. The teeth appear both normally formed.
Apparently, dicephalia (two-headed) is rare in sharks (or rarely reported anyway). Besides the bullshark here, previous reports include the Squalus acanthias, longnose spurdog Squalus blainville, milk shark Rhizoprionodon acutus, blue shark Prionace glauca, and the tope shark Galeorhinus galeus. Total hat tip to David Shiffman, aka Mr. Shark, aka purveyor of weird, aka moncephalic scientists, aka @whysharksmatter
C. M. Wagner, P. H. Rice and A. P. Pease First record of dicephalia in a bull shark Carcharhinus leucas (Chondrichthyes: Carcharhinidae) foetus from the Gulf of Mexico, U.S.A. Journal of Fish Biology 25 MAR 2013 | DOI: 10.1111/jfb.12064