Large intervals in the clone lattice
  We give three examples of large intervals in the lattice of (local) clones on an infinite set X, by exhibiting clones C_1, C_2, C_3 such that:   (1) the interval [C_1, O] in the lattice of local clones is (as a lattice) isomorphic to {0,1,2, ...} under the divisibility relation,   (2) the interval [C_2, O] in the lattice of local clones is isomorphic to the congruence lattice of an arbitrary semilattice,   (3) the interval [C_3, O] in the lattice of all clones is isomorphic to the lattice of all filters on X.   These examples explain the difficulty of obtaining a satisfactory analysis of the clone lattice on infinite sets.   In particular, (1) shows that the lattice of local clones is not dually atomic. 
Martin Goldstern, Saharon Shelah