Although it contains some of the Smithsonian's most venerable objects, the Natural History Museum is one of the least architecturally exciting buildings on the Mall. This is due to functionalist attitudes and economizing measures on the part of the Smithsonian rather than to the quality of the local architectural firm of Hornblower and Marshall, for whom this was their major work of public architecture. With the defeat of their fine 1904 French-inspired design, the architects focused on the central octagon, an advanced system of fireproof construction, and spacious exhibition and storage areas. Although the white granite-clad exterior of the building is a stereotypical treatment of the four-story Beaux-Arts institutional envelope, its proportional treatment and sunken basement control the mass of a building covering 4 acres.
The adoption of Charles McKim's suggestion of a dome inspired by the Roman Pantheon predicated a Corinthian portico with elaborate capitals derived from the Temple of Jupiter Stator in Rome. The central dome is buttressed by four extended gables broken by thermal windows, a motif frequently used by McKim, Mead and White. An octagonal version of such a dome was simultaneously under construction at the Army War College (see SW14, p. 241). The dome covers the building's best architectural feature, a three-story irregular octagon with a Guastavino tile vault, modeled on that of McKim, Mead and White's Low Library at Columbia University (1898). The Natural History Museum's octagon has doors and loggias set within short pier walls that make the transition from the octagon to the circular dome. Longer walls that run between them contain triple-tiered balconies for ease of circulation on each level. Three levels of columns are not the usual Doric below Ionic below Corinthian, possibly because the bottom two stories (Ionic above Doric) are the same height, and the third story (Ionic) is shorter. Indiana limestone walls and variegated marble columns were a concession to architectural grandeur for the building's main space; the remaining interior walls are covered with glazed terracotta tiles.
Steel frame construction allowed for three 116-foot-wide skylit main halls with no permanent interior walls, thus maximizing flexibility of exhibition areas. The second-floor north hall on the main axis, the present Hall of Dinosaurs, is the only area today where this dramatic span is still visible. The diameter of the octagonal hall is 81 feet 6 inches along its main axis. The dome's diameter is 75 feet and its total height is 165 feet.