The '''Primogenitor Chapters''' , or '''First Born''', are those the [[Second Founding]] [[Space Marine Chapter]]s known to have been split descended from the [[Ultramarines]] after the [[Horus HeresySpace Marine Legion]].{{Fn|1b}} The Primogenitor Chapters revere [[Roboute Guilliman]] in what was known as the , [[Second FoundingPrimarch]]of the Ultramarines, as their Patron and founding father. {{Fn|1b}}  These Chapters have stayed close-knit, often fighting alongside each other and always standing ready to answer any call from the [[Chapter Master]] of the Ultramarines.{{Fn|1}}{{Cite This}}

The exact number of Primogenitor Chapters is unknown.{{Fn|1b}} The [[Apocrypha of Skaros]], the earliest known copy of the [[Codex Astartes]], lists that there are 23, but does not name them.{{Fn|1b}} *[[Aurora Chapter]]{{Fn|1a}}{{Fn|2}}*[[Black Consuls]]{{Fn|1a}}{{Fn|2}}{{Fn|3}}*[[Doom Eagles]]{{Fn|1a}}*[[Eagle Warriors]]{{Fn|1a}}{{Fn|2}}*[[Genesis Chapter]]{{Fn|1a}}{{Fn|2}}{{Fn|3}}*[[Inceptors]]{{Fn|1a}}{{Fn|2}}*[[Libators]]{{Fn|1a}}{{Fn|2}}* [[Mortifactors]]{{Fn|3}}*[[Novamarines]]{{Fn|1a}}{{Fn|2}}{{Fn|3}}* [[Nemesis Chapter]]{{Fn|2}}*[[Patriarchs of Ulixis]]{{Fn|1a}}{{Fn|2}}*[[Praetors of Orpheus]]{{Fn|1a}}{{Fn|2}}*[[Silver Eagles]]{{Fn|1a}}{{Fn|2}}*[[White Consuls]]{{Fn|1a}}{{Fn|2}}{{Fn|3}}

*{{Endn|1}}: [[Codex: Ultramarines (2nd Edition)]]:** {{Endn|1a}}: pg. 9** {{Endn|1b}}: pg. 13* {{Endn|2}}: [[Chapter Approved 2001]], pg. 9117* {{Endn|3}}: [[Deathwatch: Rites of Battle]], pgs. 55-58