What is an "amtrac" as noted in the nonfiction book, With the Old Breed, at Peleliu and Okinawa?

An amtrac was an amphibious tractor used by the marines for the amphibious assault on Peleliu and Okinawa, among others. They were medium-sized armored vehicles, often mounting weaponry as large as 75mm cannon, and capable of crossing land on their tracked wheels or moving through water. Their capability of moving from the open sea to the landing beaches by "crawling" across coral reefs, was particularly important to the amphibious assault tactics used by the marines in the Pacific Theater. Several photographs of amtracs are presented in the photographic plates in the text.