When the nitrogenous base forming the nucleoside is pyrimidine, it is called a pyrimidine nucleoside. The 1st carbon of the pentose sugar is joined to the 1st nitrogen of the base by a glycosidic bond to form a pyrimidine nucleoside. It is of three types. Thymidine nucleoside, cytidine nucleoside and uridine nucleoside. Pentose sugars and thymine combine to form thymidine nucleosides, pentose sugars and cytosine combine to form cytidine nucleosides and pentose sugars and uracil combine to form uridine nucleosides.