with relatively high optical depth in the more abundant isotopologue. We do see that when an average over $\sim$ 10$^6$ pixels of mask 1 is formed, we readily see emission in  as well as . The ratio of peak intensities is significantly larger in mask 1 than in mask 2. The value, about 10, is still much less than the presumed abundance ratio \[\]/\], suggesting that the in mask 1, while optically thick, typically has lower opacity than in mask 2.

The mask 0  and  spectra show two or three peaks, including velocities for which the emission in mask 2 is very weak compared to that in the range of the peak emission, 5  to 8 . In particular, the 10  emission feature comes from a fairly extended region in the northern portion of our map, but is so weak that only when averaging over modest-sized ($\sim$1 square degree) regions in mask 0 can it be detected. Emission in this velocity range can be quite clearly seen in the mask 1 spectrum, but hardly can be detected in mask 2. This is consistent with it being relatively low average column density material, which is extended over quite large areas. Thus, 