A Fairfax County Juvenile Court judge dismissed a murder charge against 15-year-old Larry Viscidi yesterday, ruling that only enough evidence has been presented to charge him with involuntary manslaughter in the death of his 12-year-old brother Billy.

Judge Thomas A. Fortkort ruled after a closed four-hour hearing that the prosecution failed to present evidence of malice necessary to sustain a murder charge in Billy's death, according to Commonwealth's Attorney Robert F. Horan Jr.

Involuntary manslaughter, a felony, is punishable by up to five years in prison.

Billy Viscidi was reported missing July 25, but his body was not discovered, buried in the back yard of the Viscidi home in Vienna, until Aug. 12.His brother Larry was arrested Sept. 13. Horan has said Billy died of a blow to the back of his head that caused "a horrendous fracture."

Following yesterday's preliminary hearing, Larry Viscidi, accompanied by his parents, left the courthouse and walked past a crowd of camerament while wearing a large brown paper shopping bag over his head.

A court bailiff said the youth's parents, Burton and I. Grace Viscidi, requested the shopping bag because they were "a little leery of having the boy's picture splashed around." A hole cut in the bag both permitted the youth to see and reporters to see his face.

As in earlier juvenile court proceedings in the Viscidi case, all the attorneys and court officials involved refused to refer to Larry Viscidi by name because of the confidentiality provisions of Virginia juvenile law.

"The juvenile charged in the Vienna murder case," as Larry Viscidi was referred to yesterday, now faces a hearing Nov. 16 to determine whether he will be tried as an adult for involuntary manslaughter, said Vincent M. Picciano, director of juvenile court services.

Before that hearing the juvenile court will conduct a "social investigation" of the youth and that report will be given to prosecutor Horan. Horan said he will decide whether to ask that the youth be tried as an adult after he reads the report.

If the juvenile courts retains jurisdiction in the case and determines that Larry Viscidi was "not innocent" in his brother's death, the court could assign him to one of a number of programs aimed at rehabilitating juvenile offenders. The court could retain jurisdiction over the youth until he is 21, when his record as a juvenile offender would be destroyed.

Yesterdays' hearings, to determine whether there was "probable cause" to prosecute Larry Viscidi, was extraordinarily long, Picciano said.

He said this was because testimony of several witnesses and extended legal arguments by the prosecution and defense lawyers over whether sufficient evidence had been presented to justify a murder charge.

Louis Koutoulakos, one of the defense attorneys at the hearing, said the defense presented no evidence, arguing only that the evidence presented by Horan failed to establish that a murder had been committed. He said a common language definition of involuntary manslaughter is "accidental death," and "there was no intention to harm anybody."

Horan said he sought a murder charge against "the juvenile" because "we believed that there was evidence for proving malice." He defined malice as "with a corrupt or vicious motive."

Both the prosecution nor the defense have refrained from describing publicly what they believe to be the facts surrounding Billy Viscidi's death or the manner in which he suffered the skull fracture.

Witnesses in the hearing included Dr. James Byer, the county medical examiner, Phil Grimes, a Fairfax police detective in charge of the Viscidi case, and an unidentified black-haired woman and her young daughter.

The woman and her daugher refused to give their names to reporters and Picciano said only that they were prosecution withnesses.

Picciano said the defendant will remain in the custody of his parents at home under constant supervision and will not return to school.

Billy Viscidi was first thought by police to have been a kidnap victim or a runaway when his mother reported his disappearance. The day after Billy vanished, Larry told a reporter that Billy "just alked down the driveway, took a left and headed for Nutley (Road)." a major street in Vienna.

The body was found three days after Mrs. Viscidi made a tearful televised appeal for the return of Billy.