  Two new classes of skew codes over a finite field $\F$ are proposed, called
skew convolutional codes and skew trellis codes. These two classes are defined
by, respectively, left or right sub-modules over the skew fields of fractions
of skew polynomials over $\F$. The skew convolutional codes can be represented
as periodic time-varying ordinary convolutional codes. The skew trellis codes
are in general nonlinear over $\F$. Every code from both classes has a code
trellis and can be decoded by Viterbi or BCJR algorithms.
