  We specify exterior generators for $\pi_* THH(MU) = \pi_*(MU) \otimes
E(\lambda'_n \mid n\ge1)$ and $\pi_* THH(BP) = \pi_*(BP) \otimes E(\lambda_n
\mid n\ge1)$, and calculate the action of the $\sigma$-operator on these graded
rings. In particular, $\sigma(\lambda'_n) = 0$ and $\sigma(\lambda_n) = 0$,
while the actions on $\pi_*(MU)$ and $\pi_*(BP)$ are expressed in terms of the
right units $\eta_R$ in the Hopf algebroids $(\pi_*(MU), \pi_*(MU \wedge MU))$
and $(\pi_*(BP), \pi_*(BP \wedge BP))$, respectively.
