  We derive a family of entanglement monotones, one member of which turns out
to be the negativity. Two others are shown to be lower bounds on the
I-concurrence, and on the I-tangle, respectively [P. Rungta and C. M. Caves, to
appear in Phys. Rev. A]. We compare these bounds with the I-concurrence and
I-tangle on the isotropic states, and on rank-two density operators resulting
from a Tavis-Cummings interaction. Our results provide a global structure
relating several different entanglement measures. Additionally, they possess
analytic forms which are easily evaluated in the most general cases.
