George Bartlett made the most of being dropped off his first ball to score a career-best 137 as Somerset recovered from a slow start against Surrey at Guildford.
Bartlett was put down by Rikki Clarke at slip and made Surrey pay as Somerset recovered from 35 for three to make 337 on day one of the match.
Surrey openers Rory Burns and Mark Stoneman negotiated two scoreless overs before stumps and will resume the reply on Tuesday.
Somerset will return to the top of the Specsavers County Championship Division One if they win.
Adam Lyth (95) and Tom Kohler-Cadmore (83) narrowly missed out on centuries as Essex fought back strongly after tea to blunt Yorkshire's early dominance on day one of their clash at Headingley.
Third-wicket pair Lyth and Kohler-Cadmore shared 127 inside 39 overs from late morning to early evening as Yorkshire reached 224 for two, only to fall as the score slipped to 247 for five.
Essex struck four times in all after tea as Yorkshire closed on 289 for six.
Warwickshire moved to just 183 for three in 93 overs on day one of their match against Nottinghamshire at Edgbaston.
In a tussle between Division One's bottom two teams, the home side elected to bat and deployed enormous caution, most of all in an afternoon session which brought only 39 runs in 35 overs.
Dominic Sibley batted throughout the day and ended it 19 runs short of a
seventh century in nine first-class matches while he saw three of his team-mates lose their wickets.
Liam Livingstone's seventh first-class century put Division Two leaders Lancashire in a commanding position of their clash with Leicestershire at Aigburth.
Lancashire closed on 347 for seven on a day which was dominated by a sixth-wicket stand of 136 between Livingstone and Stephen Croft.
The hosts had been 102 for four at the lunch break.
Billy Root made the highest score by a Glamorgan batsman against Northamptonshire to put his side in complete command at Wantage Road.
Root's career-best 229 helped his side recover from 120 for five to close the second day on 452 for nine, a lead of 243 runs.
Alex Lees and Jack Burnham scored half-centuries to lead a solid effort with the bat for Durham against Derbyshire at Emirates Riverside.
The home side were put under pressure by fine bowling from Ravi Rampaul, who claimed figures of four for 56 for Derbyshire, producing impressive spells with the new ball in hand.
However, Lees and Burnham played the patient knocks that were demanded by head coach James Franklin in the aftermath of the club's defeat to Gloucestershire last time out.
Although neither batsmen could make three figures, Durham ended the first day in a decent position at 254 for eight.
In the other game in the second tier, Stiaan Van Zyl's marathon hundred set Sussex on course for a maximum-points win over Middlesex on day two at Lord's.
The South African left-hander batted for just over seven and a half hours for his 173 and, with wicketkeeper Ben Brown weighing in with 107, Sussex – who bowled out the hosts for 138 on the opening day – piled up 481 for nine.
Middlesex survived four overs before stumps reaching nine without loss, but they face the prospect of having to bat out almost all of the remaining two days to salvage a draw.
