We examine k -essence models in which the Lagrangian p is a function only of the derivatives of a scalar field \phi and does not depend explicitly on \phi .
The evolution of \phi 
for an arbitrary functional form for p can be given in terms of an exact analytic solution .
For quite general conditions on the functional form of p , such models can evolve to a state characterized by a density \rho scaling with the scale factor a as \rho = \rho _ { 0 } + \rho _ { 1 } ( a / a _ { 0 } ) ^ { -3 } , but with a sound speed c _ { s } ^ { 2 } \ll 1 at all times .
Such models can serve as a unified model for dark matter and dark energy , while avoiding the problems of the generalized Chaplygin gas models , which are due to a non-negligible sound speed in these models .
A dark energy component with c _ { s } \ll 1 serves to suppress cosmic microwave background fluctuations on large angular scales .