The embattled president of the University of South Carolina resigned on Wednesday, days after he delivered a commencement address marred by allegations of plagiarism and a misidentification of the school itself. In a news release, the school announced that the chairman of its board of trustees had accepted Bob Caslen's resignation, thanking him for his service. Harris Pastides, Caslen's immediate predecessor who led the system for 11 years, will serve on an interim basis during a search for a permanent replacement, officials said.