At the corner of 19th and Race streets, the Academy of Natural Sciences of Philadelphia, Pennsylvania, is the oldest continually operating museum of its kind in the Western Hemisphere. It is a natural sciences institution and was founded in 1812. In the early 19th century, Philadelphia was the cultural, commercial, and scientific center of the new nation. Classic expeditions to explore the western wilderness were organized at the academy. Explorers brought back newly discovered plant and animals that were studied and catalogued; they formed the foundation of the academy's scientific collections that now contain more than 25 million specimens. The academy opened its doors to the public in 1828. Here, the mysteries of nature were revealed. The collections expanded rapidly through gifts, purchases, and exchanges, as well as expeditions. The academy outgrew its building three times in 60 years. In 1876, its present home was built at 19th and Benjamin Franklin Parkway. By the turn of the century, academy expeditions were exploring the Arctic, Central America, and later, Africa and Asia. Plants and animals collected during those excursions became incorporated into the academy's magnificent dioramas, many of which were constructed in the 1920s and 1930s. To capitalize on the educational potential of their dioramas, the academy initiated classes for students in the School District of Philadelphia in 1932. In 1948, long before the prevention of environmental degradation became a current issue, the academy established the Environmental Research Division. One of Philadelphia's most active and progressive museums, the academy houses cases of preserved birds and animals displayed with backgrounds simulating their natural habitats. Also on display are fossils, including a collection of Thomas Jefferson's models of extinct animals, insects, and shells. The second floor features Asian and African flora and fauna. Many of the cases have headphones that permit recorded explanations of contents. Discoveries that rocked the world share four floors of exhibit space that highlight the academy’s remarkable collections. One of the largest meat-eating dinosaurs, the Giganotosaurus, towers over Dinosaur Hall. Dinosaur Hall also is the home of fossils from the Hadrosaurus foulkii, discovered in New Jersey in the summer of 1858 by fossil hobbyist William Parker Foulke. Foulke heard that 20 years previously, workers had found gigantic bones in a local marl* pit. Foulke spent the late summer and fall directing a crew of hired diggers. Eventually he found the bones of an animal larger than an elephant, with structural features of both a lizard and a bird. Youngsters can climb inside a Tyrannosaurus Rex skull, try on horns and claws, and dig for fossils. Multitudes of butterflies from Kenya, Costa Rica, and Malaysia flit around visitors in a simulated tropical rain forest. Large game animals acquired in the 1920s and 1930s are mounted in 3-D painted dioramas that replicate their natural habitats; for Philadelphians of that era, this was their first viewing of an Indian tiger or a wildebeest.