An Atlantic common dolphin washed up on Lucy Vincent Beach Monday. Tony LaCasse, spokesman for the New England Aquarium, said the recently-deceased mammal showed no signs of external injuries. He estimated the dolphin measured 7 feet, 4 inches and weighed 250 pounds. The local marine mammal stranding team and New England Aquarium were notified of the incident. Following the advice of an expert, the animal was buried on the beach. The dolphin was not among a group recently stranded off the outer Cape and later released, Mr. LaCasse said.