The brother of the third Manu, Uttama, was celebrated by the name Tāmasa, and he became the fourth Manu. Tāmasa had ten sons, headed by Pṛthu, Khyāti, Nara and Ketu.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
चतुर्थ उत्तमभ्राता मनुर्नाम्ना च तामसः । पृथुः ख्यातिर्नरः केतुरित्याद्या दश तत्सुताः
caturtha uttamabhrātā manurnāmnā ca tāmasaḥ | pṛthuḥ khyātirnaraḥ keturityādyā daśa tatsutāḥ
SB 8.1.27