The mysteries in Miss Adrian's series have been put to use as remedial entertainment for slow readers at the 4th and 5th grade levels and generally offer doses of hard information and suspense in unequal parts. (The Uranium Mystery, Mystery of the Night Explorers.) This one seems less successful than the others because there isn't very much mystery and the dialogue has that unsubtle flow of science information being introduced as three youngsters set about fossil hunting on a summer vacation in Utah. Most of the excitement comes when the children's patient guide, an elderly hobbyist, has to leave them alone at the dinosaur site after a truck tire blows out. They become lost, are found next day and eventually find bones. Flat and pat.