  We have simulated lattice QCD with 2 flavours of massless staggered quarks.
An irrelevant chiral 4-fermion interaction was added to the standard quark
action to allow us to simulate at zero quark mass. Thermodynamics was studied
on lattices of temporal extent 6. Clear evidence for a second order chiral
transition was observed and the critical exponents $\beta_{mag}$, $\delta$,
$\nu$ and $\gamma_{mag}$ were measured. These exponents did not agree with
those expected by standard universality arguments. They were, however,
consistent with tricritical behaviour. The $\pi$ and $\sigma$ screening masses
were measured and showed clear evidence for chiral symmetry restoration at this
transition.
