New Yorkers alarmed by reports over the weekend that a plague of alien snails had descended upon the Central Park lake, were reassured yesterday by William J. Clench, curator of mollusks, who identified the species as harmless and 100 percent American.

"They've probably been in the lake for thousands of years." Clench said, "but nobody ever worried about them before."

Public anxiety was aroused, he explained, because of recent scare stories abouta giant Pacific small, introduced by the Japanese into Guam and Saipan for food purposes during the war. The Pacific snails multiplied like rabbits but their danger to crops has been over-rated, Clench said.

New York authorities were at first unable to identify the Central Park snail, because part of the American Museum of Natural History's reference collection of mollusks is on loan here at Harvard. The golf ball-sized shells were later attributed to the common American fresh water group, "Viviparus conectoides."

These snails made an appearance in the Boston area several years ago when they invaded Muddy River in Brookline and were harvested and marketed by the inhabitants of Chinatown.

Last summer, Congress tightened up quarantine laws in order to keep out foreign snails, some species of which carry disease.