Mired in blue cement and clad in white Harvard sweat pants, 18 musical celebrities yesterday immortalized themselves in the sidewalk adjacent to a new Harvard Square blues club.

Movie star Dan Aykroyd presided over the festivities at the House of Blues, located on Winthrop Street across from the Spaghetti Club. Aykroyd is best known for co-starring, along with the late John Belushi, in the movie "The Blues Brothers."

The elegant new blues bar and restaurant--scheduled to open officially next week--is a cooperative venture between Aykroyd, actor River Phoenix and Isaac Burton Tigrett, founder of the Hard Rock Cafe.

Yesterday's ceremony inaugurated the House of Blues' "Blues Walk." The sidewalk tribute is designed to be analogous to Hollywood's Walk of Fame, which features the hand prints of celebrities.

"Here at the House of Blues, we are inaugurating the first driveway featuring butt prints," Aykroyd said.

Then Aykroyd and 17 other blues greats, including musicians Paul Shaffer and Joe Walsh, lowered themselves--feet first, then hands and then backsides--onto freshly laid squares of blue cement.

"I think [the club] is going to fly because it's a celebration of American media and it's a cultural artifact," Aykroyd said.

The comedian, who during the late 1970s was a cast member of "Saturday Night Live," attended yesterday's celebration dressed in his trademark black derby hat and blazer.

Parked on the street in front of the new restaurant was the battered but familiar police car that Aykroyd and Belushi drove on their "mission from God" in "The Blues Brothers."

Aykroyd was scheduled to make an appearance at the Lampoon Castle following the gathering last night, according to Lampoon members.