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\subsubsection{CO Velocities}  \label{CO}  CO observations trace gas with mean density around 100 cm$^{-3}$. CO emission associated with the Scutum-Centaurus Arm of the Milky Way \citep{Dame2011}\citep{Dame2001}  is shown in Figure \ref{fig:COarm}, which presents a plane-of-the-sky map integrated over $-50 The Judging by-eye  vertical (latitude) centroid of the CO emissionattributed to the Scutum-Centaurus Arm \citep{Dame2011}\citep{Dame2001} shown  in Figure \ref{fig:COarm} appears to follow Nessie remarkably well, even out to the full $8^\circ$ (430 pc) extent of Nessie Optimistic. This coincidence of features can be seen by-eye in Figure \ref{fig:COarm}, but we We  have also compared Nessie's shape with calculated  a trace of curve representing  the CO distribution's locus of  latitude centroid, centroids for CO in this velocity range,  and we have confirmed, at least even  at the this  coarse resolution of the CO map, that resolution,  a curve following Nessie's shape is clearly  a better fit than a straight line passing through the CO centroids. Table 1 estimates Nessie's typical ${\rm H_2}$ column density at $\gtrsim 10^{23}$ cm$^{-2}$ and its typical volume density at $\gtrsim 10^5$ cm$^{-3}$. Thus, the plane-of-the-sky coincidence of the line-of-sight-velocity-selected ``Scutum-Centaurus" CO emission and the mid-IR extinction suggests that the Nessie IRDC may be a kind of dense ``spine" or ``bone" of this section of the Scutum-Centaurus Arm, as traced by $\sim 100$ cm$^{-3}$ CO-traced gas. But, the spatial resolution of the CO map is too low ($8'$), and the 20 km\ s$^{-1}$ velocity range associated with the Arm in CO is too broad to decide based on this evidence alone whether Nessie is a well-centered ``spine" or just a long skinny feature associated with, but potentially significantly inclined to, the Scutum-Centaurus Arm.