The visitor is admitted at a door on Broad street, and ascends a flight of stairs, on the left hand as he enters the vestibule. He finds himself in a spacious saloon, one hundred and ten feet in length and forty-two feet broad, lighted from the roof and tall windows at the east and west extremities. Three ranges of galleries, supported on light and graceful iron columns, surround the apartment. The walls are hidden by glass-cases, filled almost to overflowing with specimens of natural history. Three ranges of flat cases occupy the floor, in which are arranged fossil organic remains, illustrative of that department of natural science termed palæontology. The American specimens are in the southern, and the foreign in the middle and northern range of cases; the whole constituting a collection of more than 60,000 individual specimens. Among them are some of great rarity and interest. There are several of those gigantic fish-lizards, called ichthyosaurians, imbedded in massive limestone; teeth and bones of the mastodon, of elephants, of an extinct species of bird, found in New Zealand, called the Dinornis; impressions of coal-plants, etc. etc. On the southern side of the hall is a collection of skeletons and parts of skeletons of mammals, birds, reptiles and fishes; and the extraordinary collection of human skulls, brought together here from all parts of the world, by the late Dr. Samuel George Morton, so extensively known for his publications in various departments of the history of the human race. On the northern side is a collection of mammals, representing about 200 species of the various quadrupeds. The cases on the galleries are occupied by the extraordinary collection of birds, which is three times more extensive than that of the British Museum; it contains at this time 27,000 specimens, of which no less than 22,000 are labeled and beautifully mounted, and as well displayed as the want of space will permit. Among the mammals are a specimen of the polar bear, obtained during the voyage recently made under the command of Capt. De Haven, in search of Sir John Franklin, and a fine male specimen of the Rocky Mountain sheep, a very rare animal, this being, it is believed, the second specimen ever brought to this city; the first was obtained by Capt. Lewis, during his famous expedition with Clarke to the Rocky Mountains, more than thirty-five years ago. Besides the collections alluded to, there are others of great interest which are not exhibited for want of space. The collection of crustaceans or crabs, and that of reptiles, are equal to any in Europe. The specimens of shells number 25,000; and of minerals more than 4000; but they are not at present accessible to the public for want of room to display them. The herbarium or hortus siccus, contains 46,000 species of plants.