The Collaborative International Dictionary of English v.0.48:

Skull \Skull\, n. [See School a multitude.]
   A school, company, or shoal. [Obs.]
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         A knavish skull of boys and girls did pelt at him.
   --Warner.
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         These fishes enter in great flotes and skulls.
                                                  --Holland.
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The Collaborative International Dictionary of English v.0.48:

Skull \Skull\, n. [OE. skulle, sculle, scolle; akin to Scot.
   skull, skoll, a bowl, Sw. skalle skull, skal a shell, and E.
   scale; cf. G. hirnschale, Dan. hierneskal. Cf. Scale of a
   balance.]
   1. (Anat.) The skeleton of the head of a vertebrate animal,
      including the brain case, or cranium, and the bones and
      cartilages of the face and mouth. See Illusts. of
      Carnivora, of Facial angles under Facial, and of
      Skeleton, in Appendix.
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   Note: In many fishes the skull is almost wholly cartilaginous
         but in the higher vertebrates it is more or less
         completely ossified, several bones are developed in the
         face, and the cranium is made up, wholly or partially,
         of bony plates arranged in three segments, the frontal,
         parietal, and occipital, and usually closely united in
         the adult.
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   2. The head or brain; the seat of intelligence; mind.
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            Skulls that can not teach, and will not learn.
                                                  --Cowper.
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   3. A covering for the head; a skullcap. [Obs. & R.]
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            Let me put on my skull first.         --Beau. & Fl.
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   4. A sort of oar. See Scull.
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   Skull and crossbones, a symbol of death. See Crossbones.
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WordNet (r) 3.0 (2006):

skull
    n 1: the bony skeleton of the head of vertebrates

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "skull":
   Azrael, Black Death, Death, Grim Reaper, Pale Death, Reaper,
   angel of death, brain box, brainpan, cranium, crossbones,
   epicranium, memento mori, pale horse, pale rider, pericranium,
   sickle of Death, skull and crossbones, that fell sergeant,
   that grim ferryman, white cross