  We construct merger trees for galaxies identified in a cosmological
hydrodynamical simulation and use them to characterize predicted merger rates
as a function of redshift, galaxy mass, and merger mass ratio. At z=0.3, we
find a mean rate of 0.054 mergers per galaxy per Gyr above a 1:2 mass ratio
threshold for massive galaxies (baryonic mass above 6.4E10 \Msun), but only
0.018 / Gyr for lower mass galaxies. The mass ratio distribution is
proportional to R_merg^-1.2 for the massive galaxy sample, so high mass mergers
dominate the total merger growth rate. The predicted rates increase rapidly
with increasing redshift, and they agree reasonably well with observational
estimates. A substantial fraction of galaxies do not experience any resolved
mergers during the course of the simulation, and even for the high mass sample
only 50% of galaxies experience a greater than 1:4 merger since z=1. Typical
galaxies thus have fairly quiescent merger histories.
  We assign bulge-to-disk ratios to simulated galaxies by assuming that mergers
above a mass ratio threshold R_major convert stellar disks into spheroids. With
R_major of 1:4, we obtain a fairly good match to the observed dependence of
early-type fraction on galaxy mass. However, the predicted fraction of truly
bulge-dominated systems (f_bulge} > 0.8) is small, and producing a substantial
population of bulge-dominated galaxies may require a mechanism that shuts off
gas accretion at late times and/or additional processes (besides major mergers)
for producing bulges.
