  We present new interferometric $^{12}CO$(1-0) and single-dish CO(3-2)
observations of the central parts of D478, a large (> 200 pc) dark dust cloud
located in a quiescent region of the inner disk of M31 where single-dish
CO(1-0) and CO(2-1) observations were previously obtained. Only a small
fraction (< 15%) of the CO(1-0) flux previously detected in this region with
the single-dish telescope is recorded by the interferometer. Most of the
CO(1-0) emission must therefore have the appearance of a smooth surface with
very little structure on scales smaller than approx 25'' (85 pc). Together with
the earlier CO(1-0) and CO(2-1) single-dish results the new CO(3-2) data are in
good agreement with LTE predictions for optically thick lines at Tex = Tkin =
3.5K. These results rule out the conventional model for these clouds consisting
of warm clumps with a low filling factor (as would be the case if they
resembled Galactic GMCs) and confirm that large, massive, cold molecular clouds
exist in the inner disk of M31 with kinetic temperatures close to that of the
cosmic microwave background. Such extremely low temperatures are likely to be a
consequence of the low heating rate in these particular regions of M31, where
very little massive star formation is occurring at present.
  From the CO line profile widths we estimate the Virial mass surface density
of D478 to be 80 -- 177 Msun pc-2. This is a factor 7 -- 16 times larger than
the value obtained by multiplying the CO(1-0) profile integrals with the
conventional ``X-factor''.
