Craig Overton ( 138 ) scored his maiden first - class ton .
Roelof van der Merwe ( 102 not out ) also reached three figures early on Friday as Somerset declared on 587 for eight and a lead of 249 .
Craig Overton ( 138 ) scored his maiden first - class ton and Roelof van der Merwe ( 102 not out ) also reached three figures early on Friday as Somerset declared on 587 for eight and a lead of 249 .