The author of On the Track of Unknown Animals ( a study of the abominable snowman) gives similarly serious attention to the case for the existence of sea-serpents. Dr. Heuvelmans, a zoologist, examines some 500 reported sightings (from 1693 to 1965), validates some, discredits others, concludes that there are seven distinct types of sea-serpent. Chronology of sightings, strandings, and captures.