A group of critically endangered whales was spotted lingering off the coast of Plymouth last week at a time when they are normally expected to be swimming south to warmer waters. Staff from the Plymouth-based Whale and Dolphin Conservation Society saw two or more North Atlantic right whales from Ellisville Harbor State Park on Wednesday, after a caller reported seeing them about a mile and a half off the coast. The whales are typically migrating south this time of year, and a sighting in the area is rare right now, said Regina Asmutis-Silvia, a biologist with the society. “Typically, for Cape Cod Bay that [area] is considered to be more of a late winter, early spring habitat for the whales,” she said.