Somerset reached 103 without loss but then lost four wickets either side of lunch before being rescued by a fifth-wicket partnership of 193 between Petersen and Buttler which enabled them to reach 358 for six by close.
Meanwhile, Graham Onions kept his name in the England frame with a five-wicket haul for Durham against Yorkshire at Chester-le-Street. Rain delayed the start but Yorkshire progressed serenely from their overnight 57 for three to 96 before the rot set in.
They were indebted to an unbeaten 25 from ex-Durham man Liam Plunkett in a partial recovery before they were all out for 177, 60 behind. Onions had run out Phil Jaques and taken two wickets the previous evening and when he removed the rest of the top six his figures read five for 49.
With the help of an edged four, Adil Rashid then took 12 off his 10th over of the day as he capitalised on being dropped on five and Onions finished with five for 63.
Young Sussex batsman Luke Wells hit an unbeaten 108 to help the visitors into a commanding first-innings position of 204 for two after two days of a hard-fought clash with Surrey at The Oval.
Wells batted with composure to thwart Surrey's impressive attack for over five hours and he reached his century just before the close from 234 balls and with 11 fours and a six. It was the 22-year-old's sixth century of his first-class career and his third in succession against Surrey in Championship cricket.
England Lions James Taylor and Alex Hales shared the batting honours with an Australian as Nottinghamshire ground out a first innings lead of 69 on the second day of their match against Derbyshire.
The pair both scored half centuries but the most fluent innings was played by Aussie Test opener Ed Cowan who scored 59 out of Nottinghamshire's 325 for five.
Derbyshire did not help their cause by dropping two catches but kept themselves in the contest by taking the wickets of Samit Patel (48) and Riki Wessels in the final session although Notts are well placed with Taylor unbeaten on 67.
Jimmy Adams made the highest post-war score by a Hampshire captain as his side took complete control on day two of the County Championship Division Two clash with Worcestershire at the Rose Bowl.
Resuming overnight on 151, Adams progressed to a double ton - eventually finishing 219 not out from 382 balls - as Hampshire declared on 500 for nine, their highest-ever total against Worcestershire.
The visitors were 159 for five at the close of play with both James Tomlinson and David Balcombe taking two wickets as only Moeen Ali really offered any resistance.
Half-centuries from Alex Wakely, James Middlebrook and Steven Crook put leaders Northamptonshire in control on the second day of their match against Gloucestershire at Bristol.
Wakely led the way with 88 as the visitors ran up 404 all out in reply to 192, Middlebrook making 62 and Crook 53.
Gloucestershire were left with an awkward period to bat before the close and lost skipper Michael Klinger for a single in moving to 34 for one, still 178 runs behind.
Just 71 runs were scored in 38 overs as batting proved tough for both Kent and Lancashire on a rain-hit second day of their match at Old Trafford.
Kent added just 28 to their overnight score before they were dismissed for 244 in their first innings, with Lancashire finishing on 43 for two having faced 34 overs.
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