John Gill Commentary Exodus 6:19

"And the sons of Merari: Mahli and Mushi. These are the families of the Levites according to their generations." — Exodus 6:19 (ASV)

And the sons of Merari, Mahali, and Mushi
From which sprang the families of the Mahalites, and Mushites, (Numbers 3:33):

these are the families of Levi, according to their generations:
the families that descended from him and his sons, according to the order of their birth.