Dean Elgar and Stephen Cook starred for their County sides, scoring 113 not out and 64 respectively.
Elgar played an anchor role for Somerset after his side knocked over Lancashire for 109. The Proteas opener displayed a signature knock, which saw him spend more than six hours at the crease, facing 242 balls.
Showing true grit, the 29-year-old reached his century in 205 balls, hitting 11 fours as he helped his side take a 169-run lead by putting 278 on the board.
Ryan McClaren took 4-76 for Lancashire to help restrict Somerset and put his side in to bat. Lancashire battled their way to 152-2 by stumps on day two, 17 runs behind their opponents.
Somerset vs Lancashire scorecard
Elsewhere, Cook played a leading role for Durham in their chase of 303. The 34-year-old scored 64 in an opening stand of 114 with South African-born Keaton Jennings (87). Both openers lost their wickets, as Durham trail Gloucestershire by 33 runs.
Durham vs Gloucestershire scorecard
For Kent, Wayne Parnell continued his fine form. He scored 41 to help his side post 259 in their first innings. He then took 2-46 to help bowl Derbyshire out for 159. Kent were on 142-7 by stumps on day two, holding a 242-run lead.
For Glamorgan, Colin Ingram ended day two on 84 not out, with his side trailing Leicestershire by 139. Jacques Rudolf also contributed to his side’s day two total, scoring 58 of 65 balls.
Glamorgan vs Leicestershire scorecard
Photo: Alan Martin/Action Plus via Getty Images