  We have obtained spectra of the J=2-1 and J=10-9 transitions of
cyanoacetylene (\hc3n) toward a collection of positions in the most prominent
filament, B213, in the Taurus molecular cloud. The analysis of the excitation
conditions of these transitions reveals an average gas H$_2$ volume density of
$(1.8\pm 0.7) \times10^{4} $ \cc. Based on column density derived from 2MASS
and this volume density, the line of sight dimension of the high density
portion of B213 is found to be $\simeq$ 0.12 pc, which is comparable to the
smaller projected dimension and much smaller than the elongated dimension of
B213 ($\sim$2.4 pc). B213 is thus likely a true cylinder--like filament rather
than a sheet seen edge-on. The line width and velocity gradient seen in \hc3n
are also consistent with Taurus B213 being a self-gravitating filament in the
early stage of either fragmentation and/or collape.
