A former volunteer worker for the Smithsonian Institution was convicted here today on a charge of possessing a pair of five-million-year-old fossils that had been stolen from the Museum of Natural History.

U.S. District Court Judge Alexander Harvey found Calvin Francis Allison Jr., a 25-year-old New Carrollton resident, guilty of possessing stolen government property valued in excess of $100.

One fossil shows the nearly complete skeletal structure of a stingray, minus only part of its tail, The other is a tooth from a freat White Shark, the sea killer immortalized in the movie "Jaws." That shark species is now extinct.

The two fossils disappeared from, the collection room of the Natural History Museum along with about 1,000 smaller and less rare fossils between December 1976 and May 1977, according to Kevin Foley, an FBI agent who investigated the thefts. The collection room holds 99 percent of the museum's acquisitions that are not on display.

According to testimony during the one-day, non-jury trial, Allison sold the two fossils to a foolish dealer for about $300. The dealer testified he sold another stingray fossil he had bought from Allison to a man in Japan for about $1,000.