Crat[ae]gus tomentosa

from The Collaborative International Dictionary of English v.0.48

Blackthorn \Black"thorn`\, n. (Bot.)
      (a) A spreading thorny shrub or small tree ({Prunus
          spinosa}), with blackish bark, and bearing little
          black plums, which are called sloes; the sloe.
      (b) A species of {Crat[ae]gus} or hawthorn ({Crat[ae]gus
          tomentosa}). Both are used for hedges.
          [1913 Webster]