A TEENAGER was shot to death on a New York subway platform for his flashy jewelry, although his mother believes the reason was his even flashier Christmas gift - the gold-and-diamond bridgework in his mouth. . . . PRINCESS KIKO, the wife of Emperor Akihito's younger son, gave birth to her second child Thursday morning. The mother and 6-pound, 1-ounce baby girl are in good condition, Japan's Imperial Household Agency said. . . . THE FOOD and Drug Administration has approved the use of Lamotrigine, a new drug for epileptics whose current anti-seizure medicine doesn't work well enough.