  We present CO interferometer observations of the barred galaxy NGC 4303
(M61). This galaxy has a gas concentration at the central region and offset
ridges in the bar. Sharp velocity gradients are apparent across the ridges.
Analyses of the CO data and the newborn stellar clusters revealed in HST images
indicate the existence of unresolved molecular clouds with masses of
10^4-6Msun. The observed shear velocity gradient across the ridges is too small
to break up giant molecular clouds. Therefore, the clouds are likely to survive
passage through the ridges. We discuss a cloud orbit model in a bar potential.
The model reproduces the narrow offset ridges and sharp velocity gradients
across the ridges in NGC 4303. We discuss cloud-cloud collisions (and close
interactions) as a possible triggering mechanism for star formation. The
newborn stellar clusters in NGC 4303 are located predominantly at the leading
sides of the offset ridges, where cloud orbits are densely populated and
suggest a high collisional frequency and possibly a high rate of triggered star
formation. Cloud-based dynamics is less dissipative than smooth hydrodynamic
models, possibly extending the timescales of gas dynamical evolution and gas
fueling to central regions in barred galaxies.
