extinct genus of cephalopods (animals related to the modern octopus, squid, and nautilus) found as fossils in marine deposits. Because Scaphites is restricted to certain divisions of Cretaceous time (the Cretaceous Period lasted from 144 to 66.4 million years ago) it is a useful index, or guide, fossil. Its shell form and manner of growth are quite unusual. At first, the shell in Scaphites is tightly coiled; later, it grows in a straight fashion but then coils again at its terminus, and the mature shell takes the form of a double loop linked by a straight segment.