The visitor's way now lies westward into the second compartment of the northern zoological gallery; for in this room, as in the rooms through which he has already passed, he should confine his attention, for the present, to the wall cases, reserving the examination of all table cases for his return visit, on his way out. And here the visitor may well pause to think upon the zoological travels he has already made, from the mammalia, which present the highest types of animal life; through the sub-families of birds, which form Cuvier's secondary class of vertebrata, or animals with a back-bone; to the threshold of the room in which the tertiary class of back-boned animals are deposited. This class includes the great families of REPTILES, of which there are no less than six hundred and fifty-seven varieties. Reptiles are vertebrated animals belonging to Cuvier's first great section, but distinguished from mammalia and birds, by their cold blood, their oviparous generation, and the absence of either feathers or hair from their bodies. They take precedence of fish in the animal kingdom, having lungs for aerial respiration, and "a higher circulatory organisation than the exclusive inhabitants of the water." In the museum, Cuvier's classification has been followed, with slight variations; that is to say, the reptiles have been re-divided into four classes:--the Sauria, or Lizards (in which class some modern naturalists, as Merrem and others, include serpents); the Ophidia, or Serpents; the Testudinata, or Tortoises; and the Batrachia, or Frogs. The lizards occupy the first ten wall cases in this room.