The Collaborative International Dictionary of English v.0.48:

Shark \Shark\ (sh[aum]rk), n. [Of uncertain origin; perhaps
   through OF. fr. carcharus a kind of dogfish, Gr. karchari`as,
   so called from its sharp teeth, fr. ka`rcharos having sharp
   or jagged teeth; or perhaps named from its rapacity (cf.
   Shark, v. t. & i.); cf. Corn. scarceas.]
   1. (Zool.) Any one of numerous species of elasmobranch fishes
      of the order Plagiostomi, found in all seas.
      [1913 Webster]

   Note: Some sharks, as the basking shark and the whale shark,
         grow to an enormous size, the former becoming forty
         feet or more, and the latter sixty feet or more, in
         length. Most of them are harmless to man, but some are
         exceedingly voracious. The man-eating sharks mostly
         belong to the genera Carcharhinus, Carcharodon, and
         related genera. They have several rows of large sharp
         teeth with serrated edges, as the great white shark
         (Carcharodon carcharias or Carcharodon Rondeleti)
         of tropical seas, and the great blue shark
         (Carcharhinus glaucus syn. Prionace glauca) of all
         tropical and temperate seas. The former sometimes
         becomes thirty-six feet long, and is the most voracious
         and dangerous species known. The rare man-eating shark
         of the United States coast (Carcharodon Atwoodi) is
         thought by some to be a variety, or the young, of
         Carcharodon carcharias. The dusky shark
         (Carcharhinus obscurus) is a common species on the
         coast of the United States of moderate size and not
         dangerous. It feeds on shellfish and bottom fishes.
         [1913 Webster]

   Note: The original 1913 Webster also mentioned a "smaller
         blue shark (C. caudatus)", but this species could not
         be found mentioned on the Web (August 2002). The
         following is a list of Atlantic Ocean sharks:
         * * * * * * * * * * * * * * * *
         Common and Scientific Names of Atlantic Sharks
         * * * * * * * * * * * * * * * *
         from "Our Living Oceans 1995" (published by the
         National Printing Office):
         NMFS. 1999. Our Living Oceans. Report on the status of
         U.S. living marine resources, 1999. U.S. Dep. Commer.,
         NOAA Tech. Memo. NMFS-F/SPO-41, on-line version,
         http://spo.nwr.noaa.gov/olo99.htm.
         (the following list is found at at
         http://spo.nwr.noaa.gov/app5.pdf)
         (1) Pelagic Sharks
         Thresher shark (Alopias vulpinus)
         Bigeye thresher (Alopias superciliosus)
         Oceanic whitetip shark (Carcharhinus longimanus)
         Sevengill shark (Heptrachias perlo)
         Sixgill shark (Hexanchus griseus)
         Bigeye sixgill shark (Hexanchus vitulus)
         Shortfin mako (Isurus oxyrinchus)
         Longfin mako (Isurus paucus)
         Porbeagle (Lamna nasus)
         Blue shark (Prionace glauca)
         (2)Large Coastal Sharks
         Sandbar shark (Carcharhinus plumbeus)
         Reef shark (Carcharhinus perezi)
         Blacktip shark (Carcharhinus limbatus)
         Dusky shark (Carcharhinus obscurus)
         Spinner shark (Carcharhinus brevipinna)
         Silky shark (Carcharhinus falciformis)
         Bull shark (Carcharhinus leucas)
         Bignose shark (Carcharhinus altimus)
         Galapagos shark (Carcharhinus galapagensis)
         Night shark (Carcharhinus signatus)
         White shark (Carcharodon carcharias)
         Basking shark (Cetorhinus maximus)
         Tiger shark (Galeocerdo cuvier)
         Nurse shark (Ginglymostoma cirratum)
         Lemon shark (Negaprion brevirostris)
         Ragged-tooth shark (Odontaspis ferox)
         Whale shark (Rhincodon typus)
         Scalloped hammerhead (Sphyrna lewini)
         Great hammerhead (Sphyrna mokarran)
         Smooth hammerhead (Sphyrna zygaena)
         (3) Small Coastal Sharks
         Finetooth shark (Carcharhinus isodon)
         Blacknose shark (Carcharhinus acronotus)
         Atlantic sharpnose shark (Rhizoprionodon erraenovae)
         Caribbean sharpnose shark (Rhizoprionodon porosus)
         Bonnethead (Sphyrna tiburo)
         Atlantic angel shark (Squatina dumeril)
         [PJC]

   2. A rapacious, artful person; a sharper. [Colloq.]
      [1913 Webster]

   3. Trickery; fraud; petty rapine; as, to live upon the shark.
      [Obs.] --South.
      [1913 Webster]

   Basking shark, Liver shark, Nurse shark, Oil shark,
   Sand shark, Tiger shark, etc. See under Basking,
      Liver, etc. See also Dogfish, Houndfish,
      Notidanian, and Tope.

   Gray shark, the sand shark.

   Hammer-headed shark. See Hammerhead.

   Port Jackson shark. See Cestraciont.

   Shark barrow, the eggcase of a shark; a sea purse.

   Shark ray. Same as Angel fish
      (a), under Angel.

   Thrasher shark or Thresher shark, a large, voracious
      shark. See Thrasher.

   Whale shark, a huge harmless shark (Rhinodon typicus) of
      the Indian Ocean. It becomes sixty feet or more in length,
      but has very small teeth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Carcharhinus obscurus
    n 1: relatively slender blue-grey shark; nearly worldwide in
         tropical and temperate waters [syn: dusky shark,
         Carcharhinus obscurus]