Sub-class 1.—_Elasmobranchii._ Order 1.—_Plagiostomi_: Sharks, Rays. Order 2.—_Holocephali_: Chimæroids. Sub-class 2.—_Teleostomi._ Order 1.—_Crossopterygii_: Polypterus, Calamoichthys. Order 2.—_Ganoidei_: Sturgeon, Garpike, Amia, Polyodon. Order 3.—_Dipnoi_: Lung-fishes, e. g., Ceratodus, Protopterus, and Lepidosiren. Order 4.—_Teleostei_: Perch, Cod, Salmon, Eel. Class 5.—_Amphibia._ In the evolution of vertebrates the Amphibians represent those forms which made the transition from aquatic to terrestrial life, but have lingered near the water. Certain acquisitions gained by the Dipnoi, such as a three-chambered heart and lungs, have been accentuated and firmly established by the Amphibians. Amphibia are cold-blooded vertebrates whose larval forms always have gill arches bearing gills. In some forms the gills are retained throughout life, though the adults always possess functional lungs. Nasal sacs open posteriorly into the mouth. In existing forms there is rarely any exoskeleton. There are two occipital condyles. Unpaired fins are frequently present, though they possess no fin rays. When limbs are present they possess distinct digits and conform to the same type as those of the higher vertebrates. Heart has two auricles and one ventricle. There is an inferior vena cava. The intestine ends in a cloacal chamber, as do also the urinogenital ducts. Eggs almost always laid in the water. Often a marked metamorphosis. Order 1.—_Urodela._ Amphibia which retain the tail throughout life. The order is subdivided into: a. _Perennibranchiata_, which retain the gills throughout life: _Siren_, _Proteus_. b. _Caducibranchiata_; no persistent gills: _Triton_, _Salamandra_. Order 2.—_Gymnophiona._ Body snake-like; no feet; young with gills: _Cœcilia_. Order 3.—_Stegocephala._ Extinct forms: _Archegosaurus_. Order 4.—_Anura._ Tailless amphibia; body short, with four limbs; larvæ tailed: _Frog_, _Toad_. Class 6.—_Reptilia._ Cold-blooded, air-breathing vertebrates. Limbs usually ending in claws; limbs sometimes absent; exoskeleton of scales; ribs well developed; incomplete double circulation; heart is four-chambered in the highest forms; oviparous; no metamorphosis; embryo with an amnion and allantois. The following are some of the orders that have become extinct, viz.: a. _Theromorpha_: Mammal-like saurians. b. _Ichthyosauria_: Swimming saurians. c. _Pterosauria_: Flying saurians. d. _Dinosauria_: Colossal land saurians capable of rising and resting on the hind legs. The following are the living orders, viz.: a, _Ophidia_; b, _Lacertilia_; c, _Chelonia_; d, _Crocodilia_.