  We discuss the signatures of the transition from galactic to extragalactic
cosmic rays in different scenarios, giving most attention to the dip scenario.
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons
in the energy range $1\times 10^{18} - 4\times 10^{19}$ eV, which is caused by
electron-positron pair production on the cosmic microwave background (CMB)
radiation. The dip scenario provides a simple physical description of the
transition from galactic to extragalactic cosmic rays. Here we summarize the
signatures of the pair production dip model for the transition, most notably
the spectrum, the anisotropy and the chemical composition. The main focus of
our work is however on the description of the features that arise in the
elongation rate and in the distribution of the depths of shower maximum $X_{\rm
max}$ in the dip scenario. We find that the curve for $X_{\max}(E)$ shows a
sharp increase with energy, which reflects a sharp transition from an iron
dominated flux at low energies to a proton dominated flux at $E\sim 10^{18}$
eV. We also discuss in detail the shape of the $X_{\max}$ distributions for
cosmic rays of given energy and demonstrate that this represents a powerful
tool to discriminate between the dip scenario and other possible models of the
transition.
