 Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 by IUPAC

Official Blue Book, peptide nomenclature: paragraph defining homodetic cyclic peptides (ring entirely of amino-acid residues linked by eupeptide bonds), followed by heterodetic cyclic peptides. Cite the cyclic-peptide paragraph rather than infer that all rings are head-to-tail.

Additional nomenclature locators: Section P-65.1.7.2.1 and Appendix 2, formyl terminology.