On Monday, "Saturday Night Live" regular Molly Shannon will announce that she is leaving the 26-year-old NBC comedy series in order to pursue a film career, reports The New York Times. Shannon's best known role on the show is that of Mary Katherine Gallagher, the loser Catholic school girl, and the actress' last appearance will be Feb. 17. The Times points out that Shannon's departure proves that the New York-based show, despite its age, still serves as a springboard to Hollywood. (Not that all its grads go to the movies. Some, such as David Spade, go to prime time. At the end of last season, departures were made by Cheri Oteri, Tim Meadows and Colin Quinn.) "SNL" is still potent enough, believes NBC, that next month the network is scheduling a brief prime-time version to combat CBS's "Survivor: The Australian Outback," which is perceived as a ratings threat. A spokeswoman for Shannon told The Times that the actress' future looks good, with three movie starring roles in the works: "Wet Hot Summer," "Osmosis Jones" and "Serendipity." Meanwhile, last Saturday night, Will Farrell's parody of Janet Reno was interrupted . . . by the real Reno. The ex-attorney general caught Farrell in the act. Both wore plain blue dresses.