ANATOMY OF BATRACUIANS. 465 



gills in the young, as well as in the form of the skull, there 

 being many bony parts in the skull which resemble similar 

 parts in Batracliia. Indeed so close in some characters is the 

 approximation of the fishes to the Batrachians, that the two 

 classes have been placed in a series called Ichthyopsida. The 

 Batrachians, however, differ essentially from the fishes in hav- 

 ing the bones of the skull few, directly comparable with 

 those of reptiles, birds, and mammals, and in being jointed 

 to the vertebral column by two articular surfaces called con- 

 dyles, the first vertebra, or atlas, having two corresponding 

 articulating hollows. The limbs have the same number cf 

 subdivisions, with distinct leverage systems, as the higher 

 Vertebrates, the bones composing them being closely homol- 

 ogous. True ribs now appear. Some have persistent exter- 

 nal gills, and all have well-developed lungs. So that for the 

 first time we have the coexistence of true limbs and lungs 

 in animals which are air-breathing and move about freely on 

 land, though from passing a part of their adult life in or 

 about fresh water they are said to be amphibious. The skin 

 is usually scaleless. The circulation is incompletely double, 

 there being sometimes two auricles. Like fishes, they are 

 cold-blooded. They are mostly oviparous, a few are vivipa- 

 rous, and nearly all undergo a metamorphosis. 



To enter more into detail : The vertebrae of Batrachians 

 are in the living Proteus and allies, and in the blind-worms 

 (Apoda) biconcave ; in the salamanders and in the Surinam 

 toad (Pipa) and BomMnator they are concave behind, but 

 in the toads and frogs generally they are for the most part 

 concave in front, but vary in different parts of the spinal 

 column, some of the same individuals being biconvex and 

 others biconcave. While the vertebrae are numerous in the 

 tailed forms, in the tailless toads and frogs there are but 

 eleven, two in the coccyx, one in the sacrum, the remaining 

 eight forming the rest of the column. In the frog, Avhen 

 the tail disappears, a long, spine-like piece (Fig. 428, e) called 

 the urostyle is developed from the rudiments of a few verte- 

 brae. In the extinct ArcJtegosaurus the bodies of the verte- 

 brae are but little ossified ; in Trimerorhachis they are rep- 

 resented by the bony rings of three segments, while in allied 



