It was a rare Canadien who reacted with dismay when Toronto's Lanny McDonald sent the New York Islanders into Stanley Cup oblivion with his overtime goal Saturday night.

The Islanders had surrendered any favortism north of the border when their fans hooted the Canadian national anthem. Even more pertinent, their demise created the first playoff series since 1967 between the Maple Leafs and the Montreal Canadiens, two 1917 NHL originals.

Toronto, behind graybeard goalies Johnny Bower and Terry Sawchuk, upset the Canadiens in the last of the pre-expansion cup series. Then, for 10 straight years, Toronto struggled in vain for a semifinal berth, never until Saturday night capable of winning a best-of-seven series.

Tonight in Montreal another goalie, fuzzy-cheeked 24-year-old Mike Palmateer, carries the Leafs' hopes in a role as off-the-board underdog. Montreal had won two straight Stanley Cups and is a prohibitive favorite to make it three - the same situation as 1967, when Jean Beliveau and company were derailed.

While Canada is engrossed in its intramural warfare, American hockey fans are focusing on their own combat, one fought no less zealously for lack of legendary exploits. The Boston Bruins and Philadelphia Flyers begin their best-of-seven countdown in Boston Garden, the teams' third straight meeting in semifinal play.

The Flyers shocked Bobby Orr and his Bruins in the 1974 final, as they won their first of two Stanley Cups. In 1976, Philadelphia ripped Boston in a five-game semifinal en route to its four-game blowout by Montreal. Last year, Boston erased the Flyers in four games, then lost to Montreal in the minimum.

Here is a rundown on each series: Toronto at Montreal

Palmateer has played all nine of the team's Stanley Cup games, allowing only 16 goals, two in overtime. His goals-against average of 1.73 is a remarkable one.

The 5-foot-9, 155-pound Toronto native made one of the more memorable playoff saves Saturday night, ranking with Key Dryden's 1971 stop on Jim Pappin, when he dove from behind the net and shoved his stick across the crease to block an apparent open-net score by the Islander's Mike Bossy.

Another Toronto hero has been defenseman Ian Turnbull, whose 12 points make him the playoff scoring leader. Turnbull has been brilliant in helping to compensate for the absence of all-star Borje Salming. The Swede, whose right eyelid was cut and nose broken nine days ago, is expected back for the third game of the semifinal Saturday.

Montreal has been waiting for a week, since its five-game ouster of Detroit, and it has a history of blowing out the opposition in the opener of a playoff series. Certainly, the Leafs could be excused a onesided defeat tonight since they were extended to the limit physically in that bruising set with New York.

Toronto capitalized on rough play to harness the Islanders. It will not get away with such tactics against the big, tough, talented Canadiens. The Leafs are winless in their last eight games against Montreal. Philadelphia at Boston

Flyers' goalie Bernie Parent, after receiving valuable help from the great Jacques Plante, the team's goalie adviser, has regained his form of championship days. In seven playoff games, Parent has permitted only 14 goals, for a 2.00 mark.

Philadelphia, which finished behind the Islanders and gave up the Clarence Campbell Bowl (most points by a team in the Campbell Conference) for the first time in five years, is looking for atonement - as if a 105-point season required any. They easily wiped out Bufallo, after stumbling a bit in their preliminary series with Colorado.

Boston, a 2-1-1 loser to Philadelphia during the regular season, was the only team among the final four to sweep its quarterfinal series, putting Chicago away without mercy. There will be many bodies bouncing before this series ends.