  Four limit cycles are constructed for classes 28, 30 and 31 in Zeeman's
classification, together with the results in [5 ] for class 27, [20] for class
29 and [22] for class 26 which indicate that for each class among classes
26-31, there exist at least four limit cycles. This gives a partial answer to a
problem proposed in [22] as well as in [7].
