Does medicare cover for the extraction of teeth and the rplacement of
decayed teeth with dentures.

I think it depends on the state where you live.

I tried http://www.medicare.gov/Coverage/Home.asp,
selected a random state 'California'
and then selected 'Dental Service'

It told me the information.

http://www.cms.hhs.gov/ might help too.

Medicare doesn't cover dental extractions, nor will it cover dentures.  From 
the Medicare Benefit Policy Manual; Chapter 16, General Exclusions
From Coverage; Section 140, Dental Services Exclusion:

"Items and services in connection with the care, treatment, filling,
removal, or replacement of teeth, or structures directly supporting
the teeth are not covered."

http://www.cms.hhs.gov/manuals/102_policy/bp102c16.pdf

Medicare will not cover it, however if you sign up for a Medicaid
Managed Care plan, there is a chance that a percentage of this maybe
covered. Again, it depends on the state/metro area you live in and the
managed care companies in the area.

Answer based on personal knowledge.