One of the most unusual Christmas presents you'll find this season is offered by the Museum of Paleontology--a piece of a 40-foot Tyrannosaurus rex soon to go up in the three-story atrium of the Valley Life Sciences Building.

To date the "Own a Piece of the Rex" campaign has netted $23,400 from people who want to sponsor one of the fossil bones, says Judy Scotchmoor, director of public programs for the museum. Most of the purchases have been $25 teeth and $20 pieces of the tail called chevrons, but Associate Clinical Professor of Optometry Stephen Chun and his wife, Doris, shelled out $2,500 for a pelvis.

Chun, who got his BA and OD from Berkeley--he is the School of Optometry's Alumus of the Year for 1994--chose the pelvis as a symbol of the solid foundation in education he got at Berkeley, the result of many hours of cramming in the Biosciences Library.

Among the 546 donors are students from some 30 classrooms around the country, plus 8-year-old Simon Pentzien of Neuss, Germany, who scraped together 40 marks from his allowance to purchase a T-rex tooth.

The donations pay for the cast and mounting--the skeleton alone costs $37,000 unassembled--plus interactive computer exhibits about T-rex and other dinosaurs. The skeleton should start to go up in January, but meanwhile don't put off your Christmas shopping. Call 643-9745 for information.