[Nikon-D80] Newbie question - First generation Sigma 18-200 lens

Hi... I just picked up a first generation "Sigma 18-200 for digital
DC" lens (the one without image stabilization). Despite the fact that
the box says "For Nikon AF D" have discovered that my D80 even when
set to "AF" (auto-focus) will not auto-focus the lens. I have no
issues autofocusing with my Nikon 50mm Nikor AF lens, so there is
nothing wrong with the D80's focus motor.

Did I inadvertently purchase a manual focus-only lens or is there a
problem with the lens or how I've set my camera?

Sorry for the very basic question, and my thanks in advance for any
help that is offered.