Put a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to mill a card and return Grave-Shell Scarab from your graveyard to your hand, and then causing you to discard Grave-Shell Scarab.
Thran Vigil triggers, putting a +1/+1 counter on Benthic Biomancer.
Repeat from step 2.

Infinite self-mill

Thran Vigil

Benthic Biomancer

Grave-Shell Scarab

Activate Grave-Shell Scarab's cycling ability by paying {0} and discarding it, causing you to mill a number of cards and return Grave-Shell Scarab from your graveyard to your hand due to its dredge ability.
Repeat.

Combos with "Grave-Shell Scarab"