A dark - or, in any case, bizarre -chapter in the otherwise venerable history of the Eldridge Street Synagogue on the Lower East Side was reopened yesterday when a long-lost skull rolled out from a coal-ash pile and struck the foot of a workman.

Soon, an entire skeleton was in view. After inspection by a medical examiner, the bones were taken to the Fifth Precinct station house. The official word from the police was that they could not identify the body or determine its sex. It had apparently been in the cellar more than 30 years.

The unoffical word, according to Betty Sandler, administrative director of the Eldridge Street Project, was that ''it might be a young girl, in her late teens or early 20's, which would make for an unsavory story.''

Workers are excavating the cellar of the 102-year-old synagogue, between Canal and Division Streets, in the first phase of its restoration.