Lamar shot just 25 percent for the game despite taking an 11-5 lead during the first 3 minutes.
Tristan Thompson chipped in 11 points and 13 rebounds for Texas (6-1), which has won three straight since its lone loss of the season to Pittsburgh.
Kendrick Harris led Lamar (4-3) with 13 points.
The Longhorns built a 34-25 lead at intermission and then went on a 12-0 run in the early moments of the second period.
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