Hydrolyzes cytidine or uridine to ribose and cytosine or uracil, respectively. Has a clear preference for cytidine over uridine. Strictly specific for ribonucleosides. a pyrimidine ribonucleoside + H2O = a pyrimidine nucleobase + D-ribose Binds 1 Ca(2+) ion per monomer. Homotetramer. Belongs to the IUNH family. RihB subfamily.