Pyrimidines
'Pyrimidine' refers to the structure of the molecule whereby it is a 'six-membered pyrimidine ring'. Examples of molecules which possess this structure are the nitrogenous bases Thymine, Cytosine and Uracil (Alberts, 1994, p.61).
Reference
[Alberts,B., Johnson, A., Lewis,J., Raff,M., Roberts,K., Walter, P.,(2007) Molecular biology of the cell. 5th ed. New York : Garland Science]