The presence of a solid frame in this series of animals admits of their attaining a size which is denied to any of the others. The skeleton being organized in such a manner as to give remarkable vigour and precision to all their movements. In the vertebrated animals the nervous system is also more developed. There is, consequently, a more exquisite sensibility in them than in the classes whose history we have hitherto discussed. They possess five senses, more or less fully developed, a heart, a circulation, and their blood is red. We have now to deal with a class advanced above that of fishes, that of Reptilia, which is divided as follows:-- AMPHIBIA--(BATRACHIA, CUV.) Animals having ribs or processes, or short, slight, and free vertebræ, forming a series of separate centrums, deeply cupped at both ends, one of which is converted by ossification in the mature animal into a ball, which may be the front one, as in the Surinam Toad, _Pipa_, or the hind ones in the Frogs and Toads, _Rana_. The skin is nude, limbs digitate, gills embryonal,--permanent in some, in most lost in metamorphosis,--to be succeeded by pulmonary respiration,--or both; a heart with one ventricle and two auricles. They consist of:-- I. OPHIOMORPHA. Cæciliadæ or Ophiosomæ. II. ICTHYOMORPHA. Proteidæ or Sirens, Proteus, Newts, and Salamanders. III. THERIOMORPHA. _Aglosa_ Pipa or Surinam Toads. _Ranidæ_ Frogs. _Hylidæ_ Tree Frogs. _Bufonidæ_ Toads. CHELONIA, OR TURTLES. Distinguished by the double shield in which their bodies are enclosed, whether they are terrestrial, fresh-water, or marine. The Turtles, _Chelonia_, have the limbs natatory. Mud Turtles, _Trionyx_, } limbs amphibious. Terrapens, _Emys_, } Tortoises, _Testudo_, limbs terrestrial. LACERTILIA. Having a single transverse process on each side, single-headed ribs, two external nostrils, eyes with movable lids; body covered with horny, sometimes bony, scales. _Lacerta_--the Monitors, Crocodiles, Lizards; having ambulatory limbs. _Anguis_--Ophisaurus, Bimanus, Chalcides, Seps; limbs abortive; no sacrum. OPHIDIA. Having numerous vertebræ with single-headed hollow ribs, no visible limbs, eyelids covered by an immovable transparent lid; body covered by horny scales. It includes:-- _Viperinæ_--the Vipers and Crotalidæ. _Colubrinæ_--the Colubers, Hydridæ, and Boidæ. CROCODILIA. Teeth in a single row, implanted in distinct sockets; body depressed, elongated, protected on the back by solid shield; tail longer than the trunk, compressed laterally, and furnished with crests above. The several families are:-- _Crocodilidæ_--the Gavials, Mecistops, Crocodiles. _Alligatoridæ_--Jacares, Alligators, Caiman.[4]