LOS ANGELES — The Swiss architect Peter Zumthor has modified his grand plans to transform the campus of the Los Angeles County Museum of Art, altering the shape of his building to stretch across bustling Wilshire Boulevard and away from the neighboring La Brea Tar Pits, according to renderings provided by the museum on Tuesday. Critics of the original design had raised environmental concerns, saying it would have cast a shadow over tar pits rich with Ice Age fossils.

The proposed exhibition hall, which would replace four aging buildings if approved by Los Angeles County, has been compared to a water lily, an ink stain and a Jean Arp sculpture for its free-form, organic shape. Now it is acting more amoebalike, squeezing into a new space across Wilshire that is now a parking lot.

Visitors inside the museum could walk over Wilshire Boulevard and glance down at an expanse of the road, while drivers in cars below could look up into the perimeter of the glass-walled museum. As before, the plan calls for the entire building to be perched about 30 feet above the ground on glass cylinders.

When the museum unveiled massive models for the original Zumthor design last year, with an estimated cost of $650 million, leaders of the Page Museum at the La Brea Tar Pits asserted that the proposed structure would block the light and rainfall available to the tar pits, a site still being excavated by paleontologists.