  We investigate the formation of GMCs in spiral galaxies through both
agglomeration of clouds in the spiral arms, and self gravity. The simulations
presented include two-fluid models, which contain both cold and warm gas,
although there is no heating or cooling between them. We find agglomeration is
predominant when both the warm and cold components of the ISM are effectively
stable to gravitational instabilities. In this case, the spacing (and
consequently mass) of clouds and spurs along the spiral arms is determined by
the orbits of the gas particles and correlates with their epicyclic radii (or
equivalently spiral shock strength). Notably GMCs formed primarily by
agglomeration tend to be unbound associations of many smaller clouds, which
disperse upon leaving the spiral arms. These GMCs are likely to be more massive
in galaxies with stronger spiral shocks or higher surface densities. GMCs
formed by agglomeration are also found to exhibit both prograde and retrograde
rotation, a consequence of the clumpiness of the gas. At higher surface
densities, self gravity becomes more important in arranging both the warm and
cold gas into clouds and spurs, and determining the properties of the most
massive GMCs. These massive GMCs can be distinguished by their higher angular
momentum, exhibit prograde rotation and are more bound. For a 20 M$_{\odot}$
pc$^{-2}$ disc, the spacing between the GMCs fits both the agglomeration and
self gravity scenarios, as the maximum unstable wavelength of gravitational
perturbations in the warm gas is similar to the spacing found when GMCs form
solely by agglomeration.
